Adding "fluid" mode

This is a first pass at the ideas discussed in #982.
The original tag is really only used as an embed code. In some cases it's destroyed and the html5 tech creates a new one, so this wouldn't work in those cases. What we need to do here is make the [player's videoWidth/videoHeight methods](https://github.com/videojs/video.js/blob/055d81dc3abc74c7a4dc88801c95cee67d2ead04/src/js/player.js#L1548) work, even if it's just for html5 video. In techs that don't support it, we could just return the player width().

A videoWidth method would also need to be added to the HTML5 tech, and it could look just like most of [the other properties](https://github.com/videojs/video.js/blob/055d81dc3abc74c7a4dc88801c95cee67d2ead04/src/js/media/html5.js#L189).