Allow posting via micropub with markdown

[ngm]

This post, for example, I formatted with markdown markup, but when I post via micropub to wordpress I just get the plain text. It would be nice to get a way for the markdown to be processed and rendered as html.

[ngm]

Some notes on this:
There are some criticisms of Markdown, see here (https://indieweb.org/markdown) and here (http://tantek.com/w/Markdown)

But the basic principal of lightweight markup seems pretty good. Tantek's version is good.