adhere to the XDG spec regarding where to place config

Keep config stuff out of the homedir on Linux.

This also improves the homedir detection: https://github.com/sindresorhus/user-home/blob/ab23b9ba99474e22b8bfa29cebbbc57c7eaa2627/index.js

XDG spec: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html