Allow saving of Daily Mixes

Much like the current `save-weekly` and `save-daylist`, it would be cool to save any of the six Daily Mix playlists Spotify generates. This would probably combine the logic behind the scene in the two `save` commands, as well as using the fuzzyfinder logic in `spc switch` to show a list of all of the Daily Mix playlists to save, including in the list the number of the playlist (Daily Mix 1, Daily Mix 2, etc) and the list of artists included in the Daily Mix.