Allow Sprite to specifiy staring animation

[straker]

Currently the starting animation of a Sprite is the first animation listed. If you want the Sprite to start with a different animation you have to call playAnimation right after creating the Sprite.

const sheet= SpriteSheet({
  animations: {
    idle: ...,
    walk: ...
  }
})

const sprite = Sprite({
  animations: sheet.animations
});
sprite.playAnimation('walk');

It would be nice if there was an option in the Sprite constructor to specify the starting animation.