Respect XDG Base Directory spec

Try to find the configuration file in $XDG_CONFIG_HOME/peco/ or ~/.config/peco/ in accordance with the [XDG base directory specification](http://standards.freedesktop.org/basedir-spec/latest/ar01s03.html) before trying ~/.peco/config.json.

I'm quite new to Go, so feel free to let me know if there's anything wrong or that's not quite idiomatic and I'll fix it up.