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.
In the really nit-picking department, maybe:
```python
response, _ = self._music_lib_search(sonos_playlist.item_id, 0, 1)
update_id = response['UpdateID']
```
makes it a bit more explecit and thereby improves readability.