Yet another iteration of config management: we now always output
CustomOreGen_Config_Default.xml. User should copy that to
CustomOreGen_Config.xml to modify. This solves the problem of not
exporting the latest version of the config, and it's more consistent,
in that CustomOreGen_Config.xml always corresponds to a "custom"
config, whether it is global, per-save, or per-save-dimension. It
should be somewhat obvious to the user that the _Default variant is
just the defaults.