Multiple Snakes

Added ability to add multiple snakes.
Press S to add a snake. Press F to activate pathfinding (A*) on all
snakes.
Still a few bugs:
- Sometimes snakes randomly walk through walls?
- When a snake is under a wall and a cookie gets eaten, the snake tries
to pathfind to the next one which doesn't work since the snake is under
wall.
- Walls may get added on top of a snake
- Snake body parts don't follow the snake off screen to other side
- Collision with snakes is mediocre? But it works.
- Needs graphics