feat: parse Label element

[RomeroDiver]

This is a possible fix for videojs/video.js#6960

[gkatsev]

Suggested change

	assert.ok(label in output.mediaGroups.AUDIO.audio);
	assert.ok(label in output.mediaGroups.AUDIO.audio, 'a label exists');
	assert.equal(output.mediaGroups.AUDIO.audio.label, label, 'we got the label set in the mpd');

[RomeroDiver]

I'm not sure this is the correct assertion, the second one. There is no property label on audio object.
The structure of the audio object is similar to

{
  audio: {
    [label]: {...}
  }
}

[gkatsev]

Thanks for PR, just one small addition to the test, code looks good!

[slasktrat]

+1 for this!