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 aware of that, but it shouldn't be a problem? Unless I understand what you're trying to do.

You're interjecting the state switch to switch to the transition state instead of directly to the new state. So a state switch needs to happen either way, the only line that's really relevant is this one, which determines the new state:

```haxe
_state = _requestedState;
```