Provide artBase URL in config to load album art correctly

[hdurdle]

The underlying node-sonos-http-api API sometimes returns relative urls for album art, which can't be loaded in all circumstances. See https://github.com/jishi/node-sonos-http-api/issues/673

This change allows you to provide a config option for a base URL for the art to load from. Put this in your MMM-Sonos config section in config.js:

artBase: 'http://10.107.0.50:1400'

where that is the IP of a Sonos device on your network. It doesn't need to be one in the group playing.

This is working for me on my install - tested with, TuneIn, Spotify, SoundCloud, Audible, Plex and the local music library.