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.
Well, I'm not sure about the control flow, but does this mean that the function could return a string that points into the middle of an allocation? It looks like the function means the caller to free the return value, so could that be a problem?