display current volume level

See gh-55

example output:
```
02:08 [37%]  [=====================>                                    ] vol: 73%
```
display current volume level with graph

See gh-55

If the locale is UTF-8, display the current volume level as a graph:
```
02:08 [37%]  [=====================>                          ] [▁▂▃▄▅▆  ]
```

else fallback on displaying current volume level as text:
```
02:08 [37%]  [=====================>                          ] vol: 73%
```
Merge pull request #56 from thomasleveil/feature/display_volume

display volume when playing with mplayer