Custom world creation; current custom worlds cannot be opened

From 18w06a: "... don't work in this snapshot. Sorry. We have plans to re-add them in a much more extendible way, but unfortunately we were not able to have that ready yet. You will be unable to open any world of type "customized" in this snapshot, it will tell you to revert to an old version to play in those worlds."

Still not supported in 1.13-pre4. Waiting patiently for custom world support.