Add a command for loop status

https://specifications.freedesktop.org/mpris-spec/latest/Player_Interface.html#Property:LoopStatus

```bash
# get loop status ("None", "Track", or "Playlist")
playerctl loop

# turn loop status off
playerctl loop none

# set loop status to track mode
playerctl loop track

# set loop status to playlist mode
playerctl loop playlist
```

Support all the other new features too.