Add option to buy items you have zero of

If I want to create a buying shop I need at least one item to hit the chest with so if I have zero I can't set up the shop. I would like to be able to pick the item from a list or something.

https://github.com/snowgears/Shop implemented it by letting you hit the chest with your hand and then locking you in creative mode where you can pick from the list of items the one you're buying. 
