Allowing custom world generation for multiworld.

[damagefilter]

To elaborate on this one:
It actually already is possible to alter the outcome of the world generation with the ChunkCreateHook.
One can override chunk data in it.

However, it's not very sophisticated and I see two options:
1: attempt to hack into the WorldGen classes instead
2: Provide tools to properly populate chunks with stuff via the chunk creation hook.

Number 2 would still be odd since, when generating world, you wanna have an overview of chunks and not only this one isolated chunk.