MediaControls now much more customizable

MediaControls can now be initialized with additional parameters to customize its appearance.

Pass an `Object` to its constructor:

```javascript
{
    isSeekBarHidden: true, // to hide the seeking bar (default: false)
    isDurationHidden: true, // to hide the current and duration time (default: false)
    extraButtons: {
        left: [ /* An array of jQuery elements, typically <button> */ ], // Will be added after the play button in order
        right: [ /* An array of jQuery elements, typically <button> */ ], // Will be added before the mute button in order
    }
}
```