Fine tune world generator to increase likelihood of risky cores (e.g. lava) for more valuable planets

[jwflory]

To increase the difficulty of a space server, the world generator engine should have a greater likelihood to have a planet with a lava core (or another dangerous material) if the shell exterior is something that could be considered a valuable resource, such as iron ore, gold ore, lapis lazuli, or other miscellaneous blocks.

This should be implemented as a configurable list in the config.yml for an administrator to determine which blocks are considered "valuable" for his or her own server.