Allow the sytemd unit to read a configuration file

Already having a service using the port 8080, I had to change it on ympd. The only way currently was to edit the systemd unit by hand, to add the `-w` option. This pull request add the ability to create a configuration file in `/etc/conf.d/ympd` instead, to set three variables (`MPD_HOST`, `MPD_PORT` and `WEB_PORT`, matching the `-h`, `-p` and `-w` options). Default values are hard-coded in the systemd unit file as I don't know any way to set them conditionally.