Parse XDG config file for user dirs

[pniedzwiedzinski]

XDG user dirs specify the user's download directory, so if this is set amfora should use these directories for saving downloaded documents.

[makew0rld]

Thanks for linking this. It looks like Amfora should be retrieving all its XDG variables from that local config file, and not checking actual env vars at all. I think I can use the xdg/userdirs package to do this.