implement webterm

[oknozor]

This is highly optional and should not be considered a priority. However It would be great to be able to embed a terminals in presentations. We could use something as simple as [class="webterm"] to fire up a terminal inside a slide.

We probably need to fork fubarnetes/webterm and adapt it to unveil (unless it works out of the box ?).

Also this would not work on static site so we need to render the terminal only when using the serve command.