Implement Weapon Powerups

[mcdenhoed]

Would be cool to have "powerup" items fly around that you can pick up.

Power up ideas:

• Bullets that shoot asteroids without dying
• Shooting 2 at once in the same direction (this could be persistent, like in star fox)
• Shooting 3 or 5 or so at slightly different angles
• Lowered cooldown
• Longer bullet time-out

[johnthagen]

I had a story based power-up idea as well. Basically you "find" Kara, perhaps in some kind of escape pod. When you pick her up, she provides some kind of a power-up. Maybe an "auto-turret" that shoots different colored bullets at nearby asteroids.

This could be demo'd before levels (#134), but ideally would mesh best with levels.
Relates to #15.

[mcdenhoed]

Initial implementation here: https://github.com/johnthagen/rust-belt/tree/powerups