Minor refactors.
Refactor arena.State to contain multiple Snake.
Add AddSnake method.
AddSnake works properly.
Modify isValidPointItemPosition to work with all snakes.
Error handling for multiple snakes.
The second snake moves!
Fix SetSnakeHeading method for multiple snakes.
Make snake.go into a two player game.
When testing two snakes, change directions for both.
update readme for 2 player snake.