Added "Hard mode" and self-hit check

[M1lkZ]

First of all, I want to mention that @fabriciobegalli inspired me to create a self-hit check. I made it based on his code that he had uploaded earlier in his pull request. Hard mode adds a button which makes the game field become smaller and walls solid so the game restarts when the snake hit it. I also made the original field size a little bigger for players comfort.