User-configurable shop

[atbrandt]

The shop should support bot owners adding additional purchase options for members. The owner should be able to define a title, description, price, and availability for the item. and purchasing the item should trigger some relatively generic functionality that allows the owner to take action after a purchase. For example, the bot could create and assign a role to the user that bought an item, which operates as a kind of "ticket" for that user, letting the server owner know that the purchase went through successfully and that the user needs their purchase redeemed.