Added fast shoot rockets

[uritsvi]

Using the keypad-a button, the player can shoot double bullet shoots that fly faster than a regular bullet.
The player gains fast bullets every time a ship is being destroyed.

[tsherif]

Thanks for your interest in space-shooter.c, @uritsvi. This looks like a cool update, but as mentioned in the README, I'm not accepting outside contributions for this project. The code is MIT licensed, though, so you're welcome to build on it however you'd like in your own fork.