volume: format_down or equivalent if sink does not exists

Just like ethernet or ipv6, a sink might not exists (example: a bluetooth device is only a sink when connected).

How about having a format_down to make the difference between a device not connected and a sink at 0%? It seems that currently only format and format_muted are supported.

Example configuration:
```
volume volatile {
format = "DEV: %volume"
format_muted = "DEV: muted (%volume)"
format_down = ""
device = "pulse:DEV_NAME"
mixer = "Master"
mixer_idx = 0
}
```

I don't know the behaviour on something else than pulseaudio but it makes sense on pulseaudio.