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.

Detail markdown plugin attributes & defaults

Expands the details already given by highlighting the defaults for all attributes and clarifying what's required.

This reverts commit b7e0d9b1.