Added the ability to allow a specific group to overstack items.

If you give a user/group the illegalstack.overstack permission, they can then possess items that are larger than the vanilla stack limit to items added to the Settings.GroupStack [] list, you can add items to this list via the config.yml or the in game config editor (/istack prot command)

for example if you add a totem to the Settings.GroupStack list, any user with the illegalstack.overstack permission will be able to hold a stack of totems of any size.

HOWEVER; if a player with this permission drops, or puts those overstacked items into a chest and another player accesses the chest those items will be removed normally. So please warn your players accordingly if you use this feature.