title: pandas describe by - additional parameters
body: <p>I see that the pandas library has a <code>Describe by</code> function which returns some useful statistics.  However, is there a way to add additional rows to the output such as standard deviation (.std) and median absolute deviation (.mad) or the count of unique values?</p>
  
  <p>I get <code>df.describe()</code> but I'm unable to find out how to add these additional summary things</p>