Sonos playlist write operations

Added new features:

  * create new Sonos' playlist
  * add queueable item to a Sonos' playlist

This is my first contribution to SoCo, I'm interested in hearing your opinion about these changes.

I tested the changes locally and everything seems to be working fine.
It seems that we can add other items that just tracks with this method, so I think we should consider renaming it. Most correctly would be `add_playableitem_to_sonos_playlist`, but that is a bit long, so maybe `add_item_to_sonos_playlist` or maybe even just `add_to_sonos_playlist` and let the rest be appearent from the method signature.