title: datefinder wont find dates when the string has ':' before date
body: <p>datefinder module doesn't find dates when there is ':' before date.</p>
  
  <p>There is a similar question here:
  <a href="https://stackoverflow.com/questions/54673878/datefinder-module-stranger-behavior-on-particular-string">Datefinder Module Stranger behavior on particular string</a></p>
  
  <pre><code>string = "Assessment Date 17-May-2017 at 13:31"
  
  list(datefinder.find_dates(string.lower()))
  #Returns [datetime.datetime(2017, 5, 17, 13, 31)]
  </code></pre>
  
  <p>However when I add : like this "Assessment Date:", it fails</p>
  
  <pre><code>string = "Assessment Date 17-May-2017 at 13:31"
  list(datefinder.find_dates(string.lower()))
  #returns []
  </code></pre>