Add the current Spotify volume to the current track info

This adds the current application volume to the output of `spotify info`.

Example:
```
Current track:
 Artist:   Neil Young
 Track:    Rockin' In The Free World
 Album:    Freedom
 URI:      spotify:track:4Y7fEQ4PAzhlLnLviRw2P4
 Duration: 4min 42s (282 seconds)
 Now at:   0min 2s
 Player:   paused
 Volume:   76
 Shuffle is on.
```