Added basic support for reveal.js.

Support unordered and ordered lists with "fragment" elements.

Modified by JGM to remove the --reveal_js-url command-line option.
Instead use -V reveal_js-url=... as with slidy and the other slide
formats.  Also cleaned up the list code in the HTML writer.
Support incremental slide view for definition lists.

All slide formats supported.
Simplified some list code.
reveal_js: nested vertical stacks used for hierarchical structure.

Variant of a patch from jamiefolson.
Results for more than one level of nesting may be odd.
Perhaps this needs revising.
Updated templates to include reveal.js.