Reworking of how sprites are flipped

* Removed "flippable"
* Added ability to flip sprites both horizontally and vertically
* Flipping based on facing direction is now customizable
* Made flipHorizontally() and flipVertically() toggle their respective flips instead of being hacky

Previously, in order to make a sprite flip horizontally when it faces left, you would put:
```
    flippable = true;
```
This has now been replaced with:
```
    facingLeftFlip = FlxSpriteFlip.FLIP_HORIZONTAL;
```