Hi!

Currently, the markdown org exporter does not export titles. For example, the
following org file:

#+TITLE: My Title
#+SUBTITLE: My Subtitle
#+OPTIONS: toc:nil

exports to an empty file. This patch adds title support to ox-md. Currently it
exports them as markdown headers. For example, the above org file will export
to:

# My Title


## My Subtitle

Of course, the title can be suppressed by adding #+OPTIONS: title:nil.

The style used for the title export can be configured using the already existing
org-md-headline-style variable.

The markdown editor should support TITLE, but I'm not sure if this is the
'correct' way to support it. It seems like the simplest way for now. If people
want it, I could add a new option to add different title export formats.

There is an existing bug report and discussion at this issue tracker:
https://github.com/larstvei/ox-gfm/issues/21.

From that issue, YAML front-matter is an option for title export, but I would
like to stay away from that (at least by default) since that dosen't seem to be
a feature in default markdown.

Feedback, suggestions, and feature requests are very much appreciated!

-Jay

PS: Apologies if any part of this email is broken, I'm attempting to switch to gnus.