Hey Org-mode enthusiasts,

Find attached a patch adding export of manual links for markdown.

I wanted to get links to man pages to work nicely for my blog 
using ox-hugo, an org exporter based on markdown, built by 
well-known org enthusiast Kaushal Modi.


Org snippet to demonstrate export to markdown feature:

--8<---------------cut here---------------start------------->8---
I sure love [[man:emacs][GNU Emacs]].
--8<---------------cut here---------------end--------------->8---

org-md-export-to-markdown before (link description missing, booh):

--8<---------------cut here---------------start------------->8---
I sure love http://man.he.net/?topic=emacs&section=all.
--8<---------------cut here---------------end--------------->8---

Export to markdown after (pretty, valid markdown, yey):

--8<---------------cut here---------------start------------->8---
I sure love [GNU 
Emacs](http://man.he.net/?topic=emacs&section=all).
--8<---------------cut here---------------end--------------->8---

This is my first contribution (I welcome feedback regarding patch 
formatting etc). No FSF papers due to contrib/.
Happy I can finally contribute /something/ back, and scratching 
that itch feels nice.

Have a fantastic day.

--
Jb