Allow custom NBT data for shops

[noahbclarkson]

Allow custom NBT data to be given for shops in shops.yml. This is beneficial as it allows custom items such as from other plugins as well as spawners etc. to be added to the shop.

Also need to find a way to make it clear that items in the shop must have the ability to be found naturally or through some way other than the shop or the price will just continually rise if its not locked.