Allow for Custom Item to be used as Currency#159

Allow for a Custom Item to be used as a shop currency.

i.e.

currencies:
custom_item:
material: SHEARS
model: 123

or

currencies:
item:
itemsAdder: “Voucher”

and have it only accept the exact nbt of the item

This can be provided by a third-party plugin - we don’t intend to add a native currency system since it is exclusively a shop plugin that utilises other currency plugins.