Support $XDG_CONFIG_HOME for config location

Saw this mentioned on HN, and love the idea.  Thanks for creating it!

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The `~/.config` location is just a fallback for when `$XDG_CONFIG_HOME` isn't set.  See [XDG basedir](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables) or scan the `git-config`/`gitattributes` manual pages for details.

Thanks,

James