Wrap nbconvert Markdown/Heading cells in live divs

Addresses https://github.com/ipython/ipython/issues/5206

This partially mimics the live IPython notebook layout, but is not a
complete solution.

IPython/nbconvert/templates/html/basic.tpl

@@ -59,6 +59,7 b' In [{{ cell.prompt_number }}]:'
 {% endblock output %}
 
 {% block markdowncell scoped %}
+<div class="cell border-box-sizing text_cell">
 <div class="input">
 {{ self.empty_in_prompt() }}
 <div class="inner_cell">
@@ -67,9 +68,11 b' In&nbsp;[{{ cell.prompt_number }}]:'
 </div>
 </div>
 </div>
+</div>
 {%- endblock markdowncell %}
 
 {% block headingcell scoped %}
+<div class="cell border-box-sizing text_cell">
 <div class="input">
 {{ self.empty_in_prompt() }}
 <div class="inner_cell">
@@ -78,6 +81,7 b' In&nbsp;[{{ cell.prompt_number }}]:'
 </div>
 </div>
 </div>
+</div>
 {% endblock headingcell %}
 
 {% block unknowncell scoped %}