support .sass themes

Shift `dependencies` to `devDependencies`.

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.