Added config options to control spawned mobs' drops.

Includes:

- An option to disable all drops from entities spawned by mob spawners

- An option to remove specific drops

- An option to add new drops (applied after the other two)

All credits for this update goes to Laike-Endaril (Pyrofab)
Nah, Pyrofab helped out too (Laike Endaril)

This was originally released as 1.6.3, but the jar had a strange build issue, so it was rebuilt, tested, found to be fixed with the new build, and re-released as 1.6.3b.