Add Markdown to the list of downloadable nbconverted formats.

Seem it was just forgotten. The list was not auto generated on purpose
IIRC, as some format, like TeX were deemed not useful in menu.

IPython/html/static/notebook/js/menubar.js

@@ -126,6 +126,10 b' define(['
             that._nbconvert('html', true);
         });
 
+        this.element.find('#download_markdown').click(function () {
+            that._nbconvert('markdown', true);
+        });
+
         this.element.find('#download_rst').click(function () {
             that._nbconvert('rst', true);
         });

IPython/html/templates/notebook.html

@@ -106,6 +106,7 b' data-notebook-path="{{notebook_path}}"'
                                 <li id="download_ipynb"><a href="#">IPython Notebook (.ipynb)</a></li>
                                 <li id="download_script"><a href="#">Script</a></li>
                                 <li id="download_html"><a href="#">HTML (.html)</a></li>
+                                <li id="download_markdown"><a href="#">Markdown (.md)</a></li>
                                 <li id="download_rst"><a href="#">reST (.rst)</a></li>
                                 <li id="download_pdf"><a href="#">PDF (.pdf)</a></li>
                             </ul>