skip back, forward

[mattieb]

Originally posted by @CosineP in #23 (comment)

I don't have any plans to implement this, but I'd like to see this use case being met as part of a larger effort:

1. Implement playback controls via keystroke or similar once the stream is running.

2. Implement skip forward to skip over the currently-played song, possibly into wait mode, possibly to play the next song.

3. Implement skip back to go backward. When we reach the beginning of the stream, start looking back into the timeline.

4. Maybe we could then implement switching of playback direction, to play in reverse? I'm not yet convinced this is valuable enough to bring into the system. But I could be convinced if someone does the legwork above and then implements reverse playback.