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'm not sure we're on the same page here, because I thought you didn't want me to touch the core Flixel FlxG.switchState() function.

My change does this:

1. If a transition needs to happen, STOP the state change from happening, START a transition instead
2. When the transition finishes, START the state change
3. #1 will be run again, but this time no transition needs to happen so state change occurs.

The transition plays by opening a substate on the current state.