Display currently playing song in modeline

[olavfosse]

This was suggested by mediapathic. I will not implement this, but I leave the issue open in case anyone else wants to have a go at it.

[olavfosse]

I think the best way to go about implementing this is to add an option along the lines of eradio-display-song-in-modeline, that when enabled utilises an external tool to fetch the song name.

The implementation needs to handle situations where a song name is not available, for example when playing a youtube stream, gracefully.