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.
This looks wrong. We always want to load the config files which we install.

Maybe we shouldn't even honor XDG_CONFIG_DIRS, because it looks like an exceptionally stupid idea and is probably only needed by things like GNOME. (I remember researching actual uses of this.)