Added support for multiple files.

Changed asteroid block name from POPULATION to ASTEROIDGROUP, to avoid namespace conflicts (thanks to NathanKell for the suggestion!). Removed support for automatically writing config files, including the auto-upgrade feature introduced in d9b76422cfa4f0c78034feb0f0f4de015b231002. The ability to introduce new config files instead of editing a single file makes auto-upgrade unneccessary, anyway.