the xdg module saves us some code and also extends configuration to
support global config files like /etc or /var/cache (in theory - in
practice, the freedesktop standards only use ~/.cache).

using --database and --config also makes more sense from the user's
perspective.

note that we also change from ~/.config/feed2exec/feed2exe.{ini,db} to
~/.config/feed2exec.ini and ~/.cache/feed2exec.db, a major "API"
change that requires manual migration.