Implement a duration method

[heff]

The media source object has a duration property. (which may have not been there when this project was started)

Having this would help prevent the Source Handler interface from getting more complicated (in-progress in videojs/video.js#1560). The HLS plugin currently overrides the vjs.Flash duration() method, whereas with this feature the HLS logic could set the duration on the media source and the video element's duration method would report the playlist duration.