Add support for Storyboard Flips etc.

I added these commands :

    sprite.additive(startTime, endTime);
    sprite.flipH(startTime, endTime);
    sprite.flipV(startTime, endTime);

I used flipH/V instead of flipX/Y as said in https://github.com/MoonShade/osu-sgl/issues/12 because it's clearer this way.

I didn't use the existing Animation class because this isn't really an animation and it didn't fit well with how the Animation class is (the startParams / endParams).