Item usability

Adds a consumable flag for items, which is true by default (although all items in the map currently have this flag explicitly defined). If it is true, the item is removed from the player's inventory on use. If it is false, the item stays and can be used as many times as the player wants.

With this in mind, we'll likely need to balance the effects so they are lower for non-consumable items since they can be used so many times.

Closes #82