VE-635: Keyboard shortcut for source editor

https://wikia-inc.atlassian.net/browse/VE-635
@BladeBronson Registering command originally was done declaratively and I don't think there is a need to change...

I think you could do something like this in this class:
```
ve.init.mw.WikiaViewPageTarget.static.surfaceCommands.push.apply(
	ve.init.mw.WikiaViewPageTarget.static.surfaceCommands,
	[ 'wikiaSourceMode' ]
);
```
Essentially you are "extending variable", which fits very well into our OO design, and you create a facility for adding more command in the future.