Add squeezebox CLI support

This code started out as an attempt to enable Squeezeboxes to play notifications, but ended up to be a replacement of the Squeezebox xPL module. I'm using this module in production now instead of that xPL module that has been running fine for many years.

The CLI interface allows us to do more than what we can do through the xPL interface, and it removed the requirement to poll the status of the various players regularly.