Mob Mask

New Features:
- add permission to allow players use mask and not receive damage from mobs
mobmask.use default to op
- add a config.yml to change the rate that the monsters will drop their heads
- added mobs to drop heads
Pigman, Spider, CaveSpider, Enderman, Blaze, Squid, IronGolem, Ghast, Pig,
Villager, Sheep, Cow, MushroomCow, Chicken, Ocelot, Lavaslime, Slime.

To-Do:
- add a command to get heads of players
- add more mobs like wolfs and rabbits...
(if found a skin with a mob that is not listed here please share)
- add a skin disguise of the player when wearing a mask if have permission mobmask.skin