Added ability to allow withers to be spawned in certain worlds

Withers can now be spawned in a world if the world is specified in the array Worlds-To-Allow-Wither-Spawn.

For worlds which are not specified to be allowed or blocked, a default value is used: Unspecified-World-Restriction-Default

I also did a couple of other things, like use Materials instead of data values, added config.yml to the github repo, and removed an extra folder.