add app loading splash screen and AppConfig (#279)

* add PyLoader class

* create global loader during app creation time and remove it when pyscript loading operations are done

* make the loader global and open/close when apps is starting. Also add concept of app config so users can set if they want to autoclose the loader of handle it themselves

* add pyconfig file

* auto add global config if there's no config set in the page

* remove changes to simple_clock example

pyscriptjs/examples/simple_clock.html

@@ -21,6 +21,8 @@
     <div id="outputDiv2" class="font-mono"></div>
     <div id="outputDiv3" class="font-mono"></div>
     <py-script output="outputDiv">
+# demonstrates how use the global PyScript pyscript_loader
+# to send operation log messages to it
 import utils
 utils.now()
     </py-script>