Move to new configuration management strategy:
* Layout is now
  CustomOreGen/{CustomOreGen_Config.xml,modules/{custom,default}}
* CustomOreGen_Config.xml is overwritten if <Config custom='false'> (default)
* The modules/default directory is always repopulated
This lets us update configurations without requiring the user to
delete them. It is also a bit cleaner.

This also fixes some bugs:
* BiomesOPlenty, Metallurgy and Factorization did not use <IfModInstalled>
* BiomesOPlenty distributions were made biome-specific (in the spirit
  of the mod)