Powerup tweaks and different screen colors

Autofire and Multi-bullet powerups now last for 60 seconds. Also, multi-bullet acts incrementally: The first collected will give you double shot, and the second triple-shot.

You can now change the terminal screen color to green (default), white or orange, by pressing [G], [W], [O] respectively at any time.

Finally, I refactored some of the code (but not too much...)