split --config and --database settings, using xdg module

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.