title: check for any missing dates in the index
body: <p>Is there any way to check for missing dates in a dataframe directly.
  I want to check if there are a missing dates between <code>2013-01-19</code> to <code>2018-01-29</code></p>
  
  <pre><code>            GWA_BTC      GWA_ETH    GWA_LTC  GWA_XLM  GWA_XRP
     Date                 
  2013-01-19  11,826.36   1,068.45    195.00    0.51    1.82
  2013-01-20  13,062.68   1,158.71    207.58    0.52    1.75
     ...
  2018-01-28  12,326.23   1,108.90    197.36    0.48    1.55
  2018-01-29  11,397.52   1,038.21    184.92    0.47    1.43
  </code></pre>
  
  <p>I tried to check it manually but it took a lot of time.</p>