title: adding filter to pandas pivot table
body: <p>I would like to add a filtering condition to a pivot table, like this:</p>
  
  <p>(Select the values of v2 equal to 'A')</p>
  
  <pre><code>pd.pivot_table(df,index=['v1'],columns=['v2'=='A'],values=['v3'],aggfunc='count')
  </code></pre>
  
  <p>Is that possible?</p>