Multiplexing

I've put together a deck with an older version of Spectacle that I've got kicking around. I'm presenting in about three weeks.

I should be able to share my screen, but multiplexing as is available in [reveal.js](https://github.com/hakimel/reveal.js/#multiplexing) would make for a good backup in case I don't have permission.

I don't have a particularly convincing case for the feature, that aside, but if it's of interest I'd love to see it happen.