feat: add commands to change snake speed

[pedromonici]

Added commands to increase(F) and decrease(L) Snake speed. Issue #33

[rogerllube]

I think it would be interesting if the snake speed was linked with the number of apples collected, instead of just a command

[monacofj]

I think it would be interesting if the snake speed was linked with the number of apples collected, instead of just a command

The user might have the option to enable or disable this feature.

[monacofj]

Added commands to increase(F) and decrease(L) Snake speed. Issue #33

Hm... I'm not sure if manually changing the snake speed during the gameplay would sound good.
Perhaps the user might choose the spread before the game starts and then go with it therefrom.

[pedromonici]

Changes have been implemented as requested: the snake's speed now adjusts as it consumes apples. This function can be enabled and disabled using the 'F' command.

[bmarquescost]

LGTM! With the addition of enabling and disabling button to this features, I think it is a great add.