Optionally show the volume level while muted

This allows users to to see when current volume level while changing the volume in muted mode.

I like being able to mute the volume and then decrease without unmuting,
so very simplified I have bindings like these:

XF86AudioLowerVolume:
```bash
	amixer set Master --$lvl
	if (muted) {
		voltune-show -m $lvl
	} else {
		voltune-show $lvl}
	)
```

XF86AudioRaiseVolume:
```bash
	amixer set Master ++$lvl unmute
	voltune-show $lvl
```
XF86AudioMute:
```bash
    voltume-show -m $lvl
```