Add support for commandline args to wt.exe (#4023)

## Summary of the Pull Request

Adds support for commandline arguments to the Windows Terminal, in accordance with the spec in #3495

## References

* Original issue: #607
* Original spec: #3495

## PR Checklist
* [x] Closes #607
* [x] I work here
* [x] Tests added/passed
* [ ] We should probably add some docs on these commands
* [x] The spec (#3495) needs to be merged first!

## Detailed Description of the Pull Request / Additional comments

🛑 **STOP** 🛑 - have you read #3495 yet? If you haven't, go do that now.

This PR adds support for three initial sub-commands to the `wt.exe` application:
* `new-tab`: Used to create a new tab.
* `split-pane`: Used to create a new split.
* `focus-tab`: Moves focus to another tab.

These commands are largely POC to prove that the commandlines work. They're not totally finished, but they work well enough. Follow up work items will be filed to track adding support for additional parameters and subcommands

Important scenarios added:
* `wt -d .`: Open a new wt instance in the current working directory #878
* `wt -p <profile name>`: Create a wt instance running the given profile, to unblock  #576, #1357, #2339
* `wt ; new-tab ; split-pane -V`: Launch the terminal with multiple tabs, splits, to unblock #756 

## Validation Steps Performed

* Ran tests
* Played with it a bunch

doc/user-docs/UsingCommandlineArguments.md

@@ -0,0 +1,156 @@
+---
+author: Mike Griese @zadjii-msft
+created on: 2020-01-16
+last updated: 2020-01-17
+---
+
+# Using the `wt.exe` Commandline
+
+As of [#4023], the Windows Terminal now supports accepting arguments on the
+commandline, to enable launching the Terminal in a non-default configuration.
+This document serves as a reference for all the parameters you can currently
+pass, and gives some examples of how to use the `wt` commandline.
+
+> NOTE: If you're running the Terminal built straight from the repo, you'll need
+> to use `wtd.exe` and `wtd` instead of `wt.exe` and `wt`.
+
+1. [Commandline Reference](#Reference)
+1. [Commandline Examples](#Examples)
+
+## Reference
+
+### Options
+
+#### `--help,-h,-?,/?,`
+Display the help message.
+
+## Subcommands
+
+#### `new-tab`
+
+`new-tab [terminal_parameters]`
+
+Opens a new tab with the given customizations. On its _first_ invocation, also
+opens a new window. Subsequent `new-tab` commands will all open new tabs in the
+same window.
+
+**Parameters**:
+* `[terminal_parameters]`: See [[terminal_parameters]](#terminal_parameters).
+
+
+#### `split-pane`
+
+`split-pane [--target,-t target-pane] [-H]|[-V] [terminal_parameters]`
+
+Creates a new pane in the currently focused tab by splitting the given pane
+vertically or horizontally.
+
+**Parameters**:
+* `--target,-t target-pane`: Creates a new split in the given `target-pane`.
+  Each pane has a unique index (per-tab) which can be used to identify them.
+  These indicies are assigned in the order the panes were created. If omitted,
+  defaults to the index of the currently focused pane.
+* `-H`, `-V`: Used to indicate which direction to split the pane. `-V` is
+  "vertically" (think `[|]`), and `-H` is "horizontally" (think `[-]`). If
+  omitted, defaults to "auto", which splits the current pane in whatever the
+  larger dimension is. If both `-H` and `-V` are provided, defaults to vertical.
+* `[terminal_parameters]`: See [[terminal_parameters]](#terminal_parameters).
+
+#### `focus-tab`
+
+`focus-tab [--target,-t tab-index]|[--next,-n]|[--previous,-p]`
+
+Moves focus to a given tab.
+
+**Parameters**:
+
+* `--target,-t tab-index`: moves focus to the tab at index `tab-index`. If
+  omitted, defaults to `0` (the first tab). Will display an error if combined
+  with either of `--next` or `--previous`.
+* `-n,--next`: Move focus to the next tab. Will display an error if combined
+  with either of `--previous` or `--target`.
+* `-p,--previous`: Move focus to the previous tab. Will display an error if
+  combined with either of `--next` or `--target`.
+
+
+#### `[terminal_parameters]`
+
+Some of the preceding commands are used to create a new terminal instance.
+These commands are listed above as accepting `[terminal_parameters]` as a
+parameter. For these commands, `[terminal_parameters]` can be any of the
+following:
+
+`[--profile,-p profile-name] [--startingDirectory,-d starting-directory] [commandline]`
+
+* `--profile,-p profile-name`: Use the given profile to open the new tab/pane,
+  where `profile-name` is the `name` or `guid` of a profile. If `profile-name`
+  does not match _any_ profiles, uses the default.
+* `--startingDirectory,-d starting-directory`: Overrides the value of
+  `startingDirectory` of the specified profile, to start in `starting-directory`
+  instead.
+* `commandline`: A commandline to replace the default commandline of the
+  selected profile. If the user wants to use a `;` in this commandline, it
+  should be escaped as `\;`.
+
+
+## Examples
+
+### Open Windows Terminal in the current directory
+
+```
+wt -d .
+```
+
+This will launch a new Windows Terminal window in the current working directory.
+It will use your default profile, but instead of using the `startingDirectory`
+property from that it will use the current path. This is especially useful for
+launching the Windows Terminal in a directory you currently have open in an
+`explorer.exe` window.
+
+### Opening with multiple panes
+
+If you want to open with multiple panes in the same tab all at once, you can use
+the `split-pane` command to create new panes.
+
+Consider the following commandline:
+
+```
+wt ; split-pane -p "Windows PowerShell" ; split-pane -H wsl.exe
+```
+
+This creates a new Windows Terminal window with one tab, and 3 panes:
+* `wt`: Creates the new tab with the default profile
+* `split-pane -p "Windows PowerShell"`: This will create a new pane, split from
+  the parent with the default profile. This pane will open with the "Windows
+  PowerShell" profile
+* `split-pane -H wsl.exe`: This will create a third pane, slit _horizontally_
+  from the "Windows PowerShell" pane. It will be running the default profile,
+  and will use `wsl.exe` as the commandline (instead of the default profile's
+  `commandline`).
+
+### Running a specific linux distro in a specific distro
+
+Say you wanted to open a "Debian" WSL instance in `C:\Users`. According to the
+above reference, you might try:
+
+```
+wt -d c:\Users wsl -d Debian
+```
+
+Unfortunately, this _won't_ work, and will give you an error about "expected
+exactly 1 argument to `--startingDirectory`, got 2". This is because we'll
+erroneously try to parse the `-d Debian` parameter as a second
+`--startingDirectory` value. This is unexpected, and is a bug currently tracked
+in [#4277].
+
+As a workaround, you can try the following commandline:
+
+```
+wt -d c:\Users -- wsl -d Debian
+```
+
+In this commandline, the `--` will indicate to the Terminal that it should treat
+everything after that like a commandline.
+
+[#4023]: https://github.com/microsoft/terminal/pull/4023
+[#4277]: https://github.com/microsoft/terminal/issues/4277