Add support for <video> tag.

Expanding this issue out to its own issue from #14 to allow discussion.

I plan on supporting the `<video>` tag in version 1.0.0 (see #122).

Currently, I plan on using the `Chewie` widget (https://github.com/brianegan/chewie) to render videos. If you have a different proposal, feel free to give your suggestion here. 