Added Reg class and documentation to template

- Moved the generic variables from `FlxG` to a new `Reg`istry class in the template.
- Added proper documentation to the `MenuState` template which should help beginners. I thought about whether this would be a good idea... I'm not sure how much control we have over the haxelib commands, but it could be nice to have an additional, optional param to disable the documentation for more experienced users, maybe `
haxelib run flixel new -name "hello_world -no-doc"`
- Also added a `super.create()` call at the end of `create()` in the `MenuState` template.

resolves #380