this way a key can be given ability to toggleHelp on and off.  Previously Reveal.showHelp could open the help screen, but no way to close it.

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

#1770, set 'data-paused-by-reveal' attribute only for the media elements, which are not already in paused state.

Closes #1603.

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.

On Ubuntu 16.04, the Logitech R400's blackout button gets mapped to 'v'
(keyCode 86). I don't know why this is, but since nothing else is
currently using 'v', this allows it to be supported out of the box.

The data-preview-link=false tag can be used to switch off preview links