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.

more in-depth documentation of slide backgrounds #1542

make theme "simple" use white titles on dark backgrounds

set/remove @disabled on navigation buttons

make speaker notes keyboard accessible

Add selection colour for Mozilla Firefox

Found that by running css-lint on the code

Places div.speaker-notes in default kbd Tab order, and when focused,
prevent slide navigation with up/down arrows, allowing up/down arrow
keys to scroll div.speaker-notes.