Persists volume changes in localStorage

It closes https://github.com/globocom/clappr/issues/292, I think it might be interesting to add domain to _keyspace_ (prefix key for localStorage), instead of having this `clappr.property` we could have this `clappr.mydomain.com.property`. Although, it also could be interesting to have every clappr with the same config, regardless its domain. What do you think?

This feature is only enabled if the client explicitly informs and for instance it only persists *volume*:

```javascript
  var player = new Clappr.Player({
    source: "http://your.video/here.mp4",
    persistConfig: true
  });
```