title: only apply date format to date columns with xlwt
body: <p>I have results from a database search where some columns have dates, but not others. I need help to write</p>
  
  <pre><code>data = [['556644', 'Mr', 'So', 'And', 'So', Decimal('0.0000'), datetime.datetime(2012, 2, 25, 0, 0), '', False, datetime.datetime(2013, 6, 30, 0, 0)],...]
  </code></pre>
  
  <p>into an Excel spreadsheet so that</p>
  
  <pre><code>easyxf(num_format_str='DD/MM/YYYY')
  </code></pre>
  
  <p>is only applied to the datetime columns. I'm quite new to Python and I've been banging my head on this for quite a few days now. Thanks!</p>