Deferring to Pandoc is not without its faults. It still requires
processing (eg. horizontal rules turning into 72 dashes). It is
significantly slower and resource hungry.
On the reverse, the markup regexps have improved over time and are able
to handle the task.
Includes:
* Moving them into an independent file
* Using named groups for clarity
* Support for multi-line list items
* Better handling of block markup at start / end of document (eg. hr)
* Better handling of the separation around block items (eg. space around a list)