XDG directory spec

Fix for #109

This is a full implementation that uses $XDG_CONFIG_HOME and $XDG_CONFIG_DIRS.
The rationale is that since CONFIG_HOME corresponds to /etc and DATA_HOME corresponds to /usr/share, everything should go in CONFIG_HOME.

This currently breaks support on Windows and Mac, so I'd like some pointers on what the config directories on those platforms are.
getenv() can return NULL, and passing NULL as argument to printf ``%s`` is undefined behavior.

Yes, it's very unlikely that HOME is not set, but it'd still be nice if it doesn't crash just because the environment is a bit unusual.