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

Fix: multiple image-urls in data-background-image

Use computed background style if colorToRgb fails (fixes #2635)