Add "Speed-Up Effect"

[alexdantas]

nSnake should behave like other snake-clones: have the speed gradually increase with time or score.

Perhaps I should add different game modes - "Arcade" for the speed-up effect and "Practice"/"Trial"/"Classic" for the current way.

Thanks, @lynus , for suggesting this on #10 .

[lynus]

hi，alexdantas，another little RFC：speed up the snake when user is holding the direction keys,which is a common effect for most snake-like games.

[ghost]

This should definitely be the case otherwise the option in the settings of Starting speed is misleading as it suggests that that speed will increase as the game goes on.

I think in most Snake games the speed increases with score, I haven't known it to do it with time. Though that could be an interesting option.