title: Extend pandas datetime index to present date
body: <p>I have a pandas dataframe with following index:</p>
  
  <pre><code>DatetimeIndex(['2013-04-16', '2013-04-17', '2013-04-18', '2013-04-19',
                 '2013-04-20', '2013-04-21', '2013-04-22', '2013-04-23',
                 '2013-04-24', '2013-04-25',
                 ...
                 '2017-07-03', '2017-07-04', '2017-07-05', '2017-07-06',
                 '2017-07-07', '2017-07-08', '2017-07-09', '2017-07-10',
                 '2017-07-11', '2017-07-12'],
                dtype='datetime64[ns]', length=406, freq=None)
  </code></pre>
  
  <p>How can I extend this index to present date i.e. 15th July, 2017?</p>