* contrib/lisp/org-md.el (org-export-md): New customize group.
(org-md-headline-style): New variable.
(org-md-headline): Allow atx and Setext style. Also provide fall-back
for low level headlines.
* contrib/lisp/org-md.el (org-md-section): # protection shouldn't
happen at the section level, when there is no difference between raw
# from export blocks and # in plain text.
* contrib/lisp/org-md.el (org-md-paragraph, org-md-plain-text): Make
sure raw # (inserted with, for example, an export snippet) don't get
escaped. A few false positives don't hurt.
