Work begun on an extended org to markdown converter

This would support fancier markdown engines. The idea was that
org-mode to Jekyll would go through markdown, rather than
HTML. However, it seems adequate and simpler to rely on org-mode's
HTML conversion engine and to simply export that for Jekyll.

Nevertheless, committing this work in case this was useful in the
future.
Avoid putting new lines after some elements.

Don't put extra newlines after each list item, source blocks, plain lists,
and org-data.