Add support for video and audio element lazy-loading via the loading attribute

Specify support for the loading attribute for the video element and audio element using similar conventions to img and iframe, where the attribute already has precedent. Loading has possible values of eager and lazy. When a media element's loading attribute value is lazy, loading of any video or poster image data, as well as autoplay playback, will defer until layout is known and the video is in the viewport.

Fixes #10376
Fixes #6636

Author: scottjehl