Allow DisplayShops to use EXP &/or other items to pay in a single store

[TonyMontero]

Short idea:

• Allow a single store to use EXPERIENCE_ORB, EXPERIENCE_LEVEL, ITEM and/or CUSTOM_CURRENCY to pay with.

Let's expand this up:

• DisplayShops is insanely powerful, but a way to increase it to make even more powerful (winning to other Shops) is the ability to implement a new currencies to pay in your store. For example, using EXPERIENCE_ORBS will allow the user to use their experience orbs to buy the item. The store owner can pick them up maybe in a new button when they come to their stores. Same happens with EXPERIENCE_LEVEL.
• Now to talk about the ITEM one we need to take a closer look to the system as it is. We can find this useful way in a plugin that allows us to create physical stores in villagers (I won't say which plugin is) but lacks a lot of systems that DisplayShops has. Since ITEM can be moved from the user's inventory, it can be even items with nbt tags stuck in them. That will allow compatibility with almost every custom physical currency item!
• The more tricky one, CUSTOM_CURRENCY, this can be powerful for APIs or even for plugins that create a lot of different currencies in the server's itself.

Final result:

• People can customize his store, me as server owner can do Server shops and let people waste his currency, items & virtual cash to progress while the users can do their own economy using new ways such as EXP levels or orbs, custom items (for a lot of plugin compatibilities) and so on. This can rocket sky the DisplayShop's utility in fact.