Drop-in replacement for the snake object

[pbignardi]

Snake object constructed as an actual object with private fields to prevent manipulation of the snake state. Future changes should deal with the modularity of the code:

• show function should enforce a theme on the snake.
• constructor should include some information on how the boundaries are identified as in #4.
• add a directSnake function to move all the logic for snake movement inside the snake object itself.

Closes #15

[pbignardi]

Concerning the boundary identification in the constructor function, this is out of the scope of the PR. No longer a drop-in replacement. Should be moved to different PR