Passive Add-In System

Addresses TeamMetallurgy/Metallurgy#47 

# Features
* Any mod, including metallurgy itself, can implement a Metallurgy plugin.
  * Plugins must implement the `rebelkeithy.mods.metallurgy.api.plugin.IPlugin` interface and have a name beginning with "Metallurgy" and ending with "Plugin".
* Plugins provide a set of `@ForgeSubscribe` event handlers that are registered to a MineCraftForge `EventBus`

----
The plugin system is currently inactive as no events are pushed to the EventBus. Adoption will require internal plugins and creation of appropriate events. Modder suggestions of plugin event use cases are welcome.

This change will be followed up with implementation of internal plugins.