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.
Previously header ids were autogenerated by the writers.
Now they are generated (unless supplied explicitly) in the
markdown parser, if the `header_identifiers` extension is
selected.
In addition, the textile reader now supports id attributes on
headers.
