Add option to choose difficulty

[osso73]

Something like doom (I suggest 3 or 4 max):

• baby / I'm too young to die
• easy / Hey, not too rough / piece of cake
• medium / Hurt me plenty / bring it on!
• hard / Ultra-Violence / I'm the boss
• nightmare / Nightmare!

Increasing difficulty means (can be chosen):

• more points to switch level
• longer speed-up
• more parts to add with each food (or at least cake)
• etc.