Update comments about markdown slide separators

They might have been correct once upon a time, but apparently not since "---" became the default.

new shortcuts; alt+arrow key skips fragments, shift+arrow key jumps to last slide in the given direction #1105

fix(highlight): allow pre fragments

prepend --r- to global style variables

When embedding reveal in a website, this will help avoid global variable conflicts with other frameworks that use global styles via variables.

Implement auto-animate id and restart

markdown plugin improvements

Add an "animateLists" option to the markdown plugin that animates all
lists by default.

Don't waste time constructing a renderer then override it if one is
passed into the options object.

Expose preview management functions