reveal.js 3.5.0 on Node.js Yarn

Changes:

Add display config option to control the display mode slide section elements. (#1324 by @mojavelinux)
Add Reveal.toggleHelp()for toggling the help overlay. (#1816 by @Frazer)
Slide fragments can now hold their own speaker notes. (#1636 by @traff)
The speaker view can now provide pacing advice. This makes it easy to detect if you're running out of time, or going too quickly. (#1564 by @aspiers, more info)
Switch to a <textarea data-template></textarea> wrapper for Markdown to avoid HTML parsing issues. (#1780 by @ccqgithub)
Background videos now autoplay in iOS. (#1851 by @astone123)
Background iframes are now interactive when there is no content in the slide or data-background-interactive is present on the slide element.
Shift dependencies to devDependencies (#1764 by @demoneaux)
MathJax is now loaded from cdnjs since cdn.mathjax.org was shut down. (#1866 by @christianp)

Bug fixes:

Fix multiple issues with autoplaying <video> and <audio>, particularly on slow connections.
Fix overview rendering issues in Firefox.