ox-md.el: Add export-block

* ox-md.el (org-md-export-block): New function.
(md): Use it.

lisp/ox-md.el

@@ -71,6 +71,7 @@ This variable can be set to either `atx' or `setext'."
 		     (comment . (lambda (&rest args) ""))
 		     (comment-block . (lambda (&rest args) ""))
 		     (example-block . org-md-example-block)
+		     (export-block . org-md-export-block)
 		     (fixed-width . org-md-example-block)
 		     (footnote-definition . ignore)
 		     (footnote-reference . ignore)
@@ -93,7 +94,6 @@ This variable can be set to either `atx' or `setext'."
 		     (template . org-md-template)
 		     (verbatim . org-md-verbatim)))
 
-
 
 ;;; Filters
 
@@ -142,7 +142,7 @@ channel."
 	    value)))
 
 
-;;;; Example Block and Src Block
+;;;; Example Block, Src Block and export Block
 
 (defun org-md-example-block (example-block contents info)
   "Transcode EXAMPLE-BLOCK element into Markdown format.
@@ -153,6 +153,11 @@ channel."
    (org-remove-indentation
     (org-element-property :value example-block))))
 
+(defun org-md-export-block (export-block contents info)
+  "Transcode a EXPORT-BLOCK element from Org to Markdown.
+CONTENTS is nil.  INFO is a plist holding contextual information."
+  (when (equal (org-element-property :type export-block) "MD")
+    (org-remove-indentation (org-element-property :value export-block))))
 
 ;;;; Headline