Add support for an XDG compliant config location

[averagebrine]

I hopped distros and forgot my keyd configuration because it was in /etc. Oops.

I've forked the repo and will work on adding a simple solution that prefers ~/.config/keyd, but will fallback to /etc/keyd. If you feel like this should be implemented, I'd love to collaborate on a PR.

Thanks!

[nsbgn]

See #767 (comment) and #45 ;) There is no per-user support at the moment

[averagebrine]

Ah, thank you!