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

* Allow slides to be served away from revealjs server location

The previous 'base' option conflicts with Grunt's 'base' option.
Changing it to 'root' avoids this. Further, updating to a newer Grunt
allows multiple parameters to be used. therefore `grunt serve --root="."
--root="pathtomyslides"` allows you to keep your slide content separate
from JS/CSS and such

* Live reload for relocated base

* Updated packages to match origin/dev

* Updated packages to match origin/dev

* Ensure root is an array

fix #1607

The autoSlide duration of media elements with data-autoplay now takes the playbackRate of said media element into account when setting the duration so that autoSlide and the media element stay in sync.