Embed support for images and videos

Would it be ok to implement RES-like embedding for images and videos? In order to stay within the privacy goals, this feature shouldn't use third-party APIs like embedly. It would also be completely opt-in, so links are never expanded by default.

If the general idea is OK, I'd like to a shot at implementing this, supporting Youtube, Imgur, Giphy and i.redd.it links.