Mob Mask

New Features:
- add mask duration thanks to IngrownPenguin
the value can be set on config.yml and that value will be the amount
of mobs that a player can kill using a mask before the mask break
- add break head message (this message is sent to the player) thinking
about broadcast to all players as when someone get a head please comment
if want this message to be sent to all players

To-Do:
- add a command to get heads of players
(if found a skin with a mob that is not listed here please share)
- add heads for zombie and skeleton horses.