Add IntelliJ project files to .gitignore
Add a simple ItemProvider interface and API

This is to allow Plugins to register custom item providers. Kits will
need an adjustment to load items dynamically for this to work.
Store kit items as a String/amount pair

This lets the ItemProvider instances do their work by creating the
items on demand, after they've gotten a chance to get registered.

This will also facilitate randomly-generated items, since a new
instance will be requested from the ItemProvider each time.