update 0.2.1

A fairly small update, but still important.

- Nerfed the shotgun: now instead of having infinite piercing on the shells, allowing it to instantly decimate massive hordes of enemies, shots now pierce one enemy, then have a 50% chance to stop on each consecutive one. It's still quite strong, but it's no longer completely busted.

- Reworked the level randomisation. Now instead of just picking a random level each time. the game keeps a list of each level you have played on that run, and won't use them.  This should reduce the chance of repeatedly getting the same level over and over.