Fixes #16; implement more amounts of worms

This implements a fix for #16 (closed). You can now select to play against 0-5 worms (respecting the player count).

Players	AIs
1	1, 2, 3, 4, 5
2	0, 1, 2, 3, 4
3	0, 1, 2, 3
4	0, 1, 2

If there is any feedback, I'd be happy to discuss and implement potential changes.