Add autoplay support for asciinema

Asciinema (https://asciinema.org/) is a popular service allowing users
to record terminal screencasts ("asciicasts") and play them back in
a react.js based player, enabling easy after-the-fact manipulation of
color themes, font size, playback speed etc.

Since asciinema/asciinema.org@2913c045, Asciinema supports an
"asciicast:play" event that enables autoplay. Fix up reveal.js in the
same way as previously for Vimeo and YouTube.