Feature transitions

See:
https://github.com/HaxeFlixel/flixel-addons/pull/97

This adds the ability to use transitions simply by calling FlxG.switchState(). It is using compiler conditionals so that there are no dependencies -- it automatically detects whether flixel_addons is in use.

(It also has a commit to fix the missing FillStyle typedef which was keeping flixel-addons from compiling)
I agree, it seems wrong to have the logic directly in `FlxG`. In my opinion, this ought to be the responsibility of either `FlxTransitionState` or some type of intermediate transition manager.