Support the XDG-Basedir specification

Improvements related to #112. Sorry for the lack of feedback on #115 at the time, I had a lot of thing on me.

This adds no new dependencies whatsoever.
I'd like to keep the original sentence as it was, and add a [footnote](http://docutils.sourceforge.net/docs/user/rst/quickref.html#footnotes) mentioning the XDG-support.