Add way to lazy load the poster preview <img> in video_tag/media_tag filters

[bberg11]

The video_tag and video_tag filters are very convenient, but they automatically generate a preview poster <img>, that we can not defer the loading of. This is especially frustrating when autoplay is true, because the image is not even seen by a user. It would be great if its loading could follow the same logic as the image_tag filter