correct grunt option usage

Remove 'history: true' from index.html

Allow background images without protocols to have query hashes/parameters.

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

Readme says the default is 'history: false', so it may be surprising
for users - it's easy to miss the contradicting setting in index.html

support <textarea data-template> `some code` </textarea>

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 #1603.

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.