wait for in-slide video/audio to load before playing, reuse same autoplay logic for slide backgrounds

Merge branch 'add-refused-to-display-tips-to-link-preview' of https://github.com/Huxpro/reveal.js into dev

Add note about head.js to readme.

Closes #1569.

Closes #1163, #1495.

The development workflow can be used with Node 4+, not just Node 6. We
should avoid showing warnings and errors when users are installing this
package via npm/yarn.

Ref. #1729, #1743.
Closes #1725.

Reduce required Node.js version to 4.0.0.

The development workflow can be used with Node 4+, not just Node 6. We
should avoid showing warnings and errors when users are installing this
package via npm/yarn.

Ref. #1729, #1743.
Closes #1725.

Hide slide number when in paper or pdf print view

Bump node engine in packages.json

Defaults in README are not current

include contributors in (c)

Implementing a better data-trim