Splitting Asteroids

In the original Asteroids, asteroids would split when taking damage. It would be nice to have a similar feature so that "chunks" of the asteroid can break off becoming a new asteroid in the scene tree.

- [ ] Asteroids may "split" when a bullet hits them. The fragments put together must make the exact form of the asteroid.
- [ ] The intial impulse should be subtle to give the appearence of a large fragment breaking away (eg, a chunk flying off in a random direction is not what i'm thinking)