Volume output when ran without arguements

instead of
 ```
$ ./volume.py 
usage: volume.py [-h] {up,down} [amount]
volume.py: error: the following arguments are required: direction
```
I'd rather see
```
$ ./volume.py 
usage: volume.py [-h] {up,down} [amount]
Current volume: 89%
```
```
$ \mpc volume
volume: 89%
```