Replace energy powerup with venom mechanic

[chriskerrc]

The energy mechanic is a bit annoying/distracting. Replace this with a venom mechanic.

• Instead of energy, the bar shows current venom ammo
• Powerup replenishes venom ammo
• This stops the venom from being overpowered, so the user can no longer blast through all the walls (effectively adds a cooldown period for venom)
• Make the venom a different colour from the enemy snake, maybe purple to match with venom bar
• When venom consumes food, need to increase snake length

[chriskerrc]

Done. Also added two venom powerups.