Updated Use Command

Should now tell the player to put something in their hand in order to
use it.
Fixed Vendor Chat and Inventory

Should properly work now.
Redesigned Custom Item Structures

Items were picked using their itemID, which previously was "Item Name"
with spaces included. This creates errors when adding custom items to
vendor inventories. So, to fix this issue, and to make the code easier
to read, these spaces in all item names were replaced with underscores,
so all custom items now read as "Item_Name".