New command to add and remove items from shops

[BrendonButler]

We need a new set of sub-commands:

/shop add <item> <buy-price> <sell-price> <max-quantity> [<quantity>] This will allow a shop owner to add items to the shop with a customer buy and sell price, a max quantity to prevent a shop from over-buying items, and an optional quantity to stock the store with from the shop owner's inventory.

Setting a negative buy price will indicate that the shop isn't selling the specified item to players. Setting a negative sell price means that the shop isn't buying the specified item from players. It will be possible to stock a store with items and have them neither purchasable nor sellable by the players.

Setting a negative quantity (if the shop owner has permission) will indicate that that item has infinite stock, meaning they can sell indefinitely.

/shop add <item> <quantity> This will allow a shop owner to stock a certain item from their inventory.

/shop remove <item> [<quantity>] This will allow a shop owner to remove an item from the shop top stop selling it and return the inventory to the shop owner's inventory (this will fail if the shop's stock is larger than the player's available inventory space).

Specifying a quantity will only remove a select number of the specified item, leaving it still available for selling and buying.

[BrendonButler]

This is now resolved, permissions will come later as its own issue GH-11