Support for MCDungeon (and potentially other offline generators)

[sshipway]

The MCDungeon project provides a script that adds procedurally-generated dungeons to an offline world.

When we have a world library , it would be good to be able to tell MCCY to take an existing saved world, and run MCDungeon (with some set parameters) on the world to add dungeons, and then save it as a new library entry.

This would probably need a new container created for the MCDungeon process that is MCCY-aware to download the word save, run with passed parameters, and then upload the updated world back to the library as a new entry ( see #83 ). MCDungeon is command-line driven so containerising it should be easy.