title: cumsum() by month but repeat the values if there is no data in that month
body: <p>I have data: <code>df</code> </p>
  
  <pre><code>    date    col1    col2
  0   1/16/2016   apple   20
  1   2/1/2016    apple   40
  2   2/2/2016    pear    60
  3   3/13/2016   apple   10
  4   5/4/2016    apple   50
  5   6/15/2016   pear    5
  </code></pre>
  
  <p>With <code>cumsum()</code> I can get cumulative sum of the values. 
  But if there is no value in a certain month, the value is not repeated.</p>
  
  <pre><code>df.set_index('date', inplace=True)
  df = df.groupby([df.index.month, 'col1']).sum()
  df['cumsum'] = df.groupby('col1')['cumsum'].cumsum()
  
  date    col1    cumsum
  Jan-16  apple   20
  Feb-16  apple   60
  Feb-16  pear    60
  Mar-16  apple   70
  May-16  apple   120
  Jun-16  pear    65
  </code></pre>
  
  <p>But I'd like to get the following result: To repeat the cumsum of <code>col1</code> values even if there is no data in that particular month. </p>
  
  <pre><code>date    col1    cumsum
  Jan-16  apple   20
  Feb-16  apple   60
  Feb-16  pear    60
  Mar-16  apple   70
  Mar-16  pear    60
  Apr-16  apple   70
  Apr-16  pear    60
  May-16  apple   120
  May-16  pear    60
  Jun-16  apple   120
  Jun-16  pear    65
  </code></pre>
  
  <p>Thanks in advance for your help.</p>