Add autoplay support for asciinema

[fghaas]

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

This patch should also apply cleanly to master.

For background, see also asciinema/asciinema-server#175.

[Delapouite]

Hi

Youtube and vimeo support also appears in the stopEmbeddedContent function just below your contribution,
Do you think asciinema should be added there as well?

[fghaas]

@Delapouite I'm afraid that wouldn't make sense. Asciinema has an API method that allows playing an asciicast, but not one that allows stopping or pausing it. See https://github.com/asciinema/asciinema.org/blob/master/app/views/api/asciicasts/show.html.slim if you want to take a look.

[bnjmnt4n]

This seems quite specific, and is better suited to a plugin IMO. YouTube and Vimeo are used by a large number of people, while I think much less people use Asciicinema. Perhaps this is better suited to a plugin.

[fghaas]

A completely unintrusive three-line patch, and you want a plugin for that? Are you sure?

[bnjmnt4n]

IMO functionality which isn't used widely shouldn't have a need to be in the core scripts. Although this might be a 3 line patch, if everyone were to submit small patches like this for every single service, the file would become extremely bloated. In the end though, it's up to @hakimel to make the decison.

[fghaas]

Look, I don't mind, I've had this in my own fork for a while anyway and don't mind keeping it that way. Also, seeing as this patch has been sitting here for 5 months I hadn't expected it to go in. Let's close this.