Respect XDG base directory structure.

This implements searching for directories in
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html,
and then falls back to ~/.peco/config.json

refs #67
Change where LocateRcfile() is definited so we can test it

refs #67
Update README
Merge pull request #71 from lestrrat/xdg-base-dir

merge xdg-base-dir to master