title: remove nbconvert --to html 'in' and 'out' prompts based on cell metadata
body: <p>I'd like to remove the red and blue 'In' and 'Out' prompts when running <code>nbconvert --to html</code>, based on cell metadata. With cell metadata such as:</p>
  
  <pre><code>{'cell_tags': {'cutcode_html': true}}
  </code></pre>
  
  <p>The following sucessfully removes the <code>'In'</code> prompt:</p>
  
  <pre><code>{% block input_group %}
   {% if cell['metadata'].get('cell_tags',{}).get('cutcode_html','') == True -%}
    &lt;div&gt;&lt;/div&gt;
   {% else %}
    {{ super() }}
   {% endif %}
  {% endblock input_group %}
  </code></pre>
  
  <p>I'd like to do the equivalent thing for the output prompt. </p>
  
  <p>There has been discussion of how to do this for <a href="https://stackoverflow.com/questions/20184994/ipython-notebook-nbconvert-how-to-remove-red-outn-text-in-top-left-hand-co">latex</a>, but I can't figure out how to do it for HTML. </p>
  
  <p>The <code>output_prompt</code> blocks for HTML don't appear to do anything, and whenever I try to make slightly modified versions of the primary templates, they won't load properly. </p>