ox-md: Correctly export footnote references

* lisp/ox-md.el (md): Do not ignore footnote references.

Footnotes do not exist in Markdown syntax, but we can fallback on
HTML.

Reported-by: Vasilij Schneidermann <v.schneidermann@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/93447>

lisp/ox-md.el

@@ -72,8 +72,6 @@ This variable can be set to either `atx' or `setext'."
 		     (comment-block . (lambda (&rest args) ""))
 		     (example-block . org-md-example-block)
 		     (fixed-width . org-md-example-block)
-		     (footnote-definition . ignore)
-		     (footnote-reference . ignore)
 		     (headline . org-md-headline)
 		     (horizontal-rule . org-md-horizontal-rule)
 		     (inline-src-block . org-md-verbatim)