Merge pull request hakimel#2651 from hakimel/dev

reveal.js 4.0.0

3.9.2 security release

3.9.1 because a local edit made it into npm

update to highlight.js 9.18.0 hakimel#2562

lab.hakim.se/reveal-js -> revealjs.com

new fix for prematurely autoplaying background videos