auto-detect insertion of video elements for JS WebP

Currently, only elements with the ewwwwebp and ewwwwebplazyload class are auto-detected when inserted into the DOM. For example, with infinite scroll, or "load more" AJAX buttons, a video element can be added to the page after the initial page parse. But JS WebP won't recognize that these new elements need to be parsed for WebP. We would need to either auto-detect videos specifically, or look into adding one of the classes to them to trigger the arrive.js routine.