title: removing certain cells via tags fails due to wrong type
body: <p>I want to hide certain parts of a jupyter notebook and came across tags which can achieve this. I've tagged the cells with <code>remove_cell</code> in my notebook an tried to run</p>
  
  <pre><code>$ jupyter nbconvert test.ipynb --TagRemovePreprocessor.remove_input_tags="{'remove_cell'}" 
  </code></pre>
  
  <p>however I always get the following error:</p>
  
  <pre><code>traitlets.traitlets.TraitError: The 'remove_input_tags' trait of a TagRemovePreprocessor instance must be a set, but a value of type 'unicode' (i.e. u'{remove_cell}') was specified.
  </code></pre>
  
  <p>I've tried to change the <code>"{'remove_cell'}"</code> to various format, e.g. <code>{'remove_cell'}</code> etc with the same result. Any help would be appreciated</p>