title: Allow null for single date in Date range picker
body: <p>I am working with <a href="https://github.com/fragaria/angular-daterangepicker" rel="nofollow noreferrer">angular-date rangepicker</a>. For my requirement I need to have the option to set date as null for start date/end date or both. I looked into its option but didn't find any. </p>
  
  <p>I also have the requirement to show checkbox in calendar popup to set <code>startDate</code> as null and checkbox for set <code>endDate</code> as null.</p>
  
  <p>I found one plunker for setting null, but it sets both date as null/blank <a href="https://plnkr.co/edit/pQRfybEeltbjQi8FSlUZ?p=preview" rel="nofollow noreferrer">Plunker</a></p>
  
  <p>The plunker uses </p>
  
  <pre><code> eventHandlers: {
        'cancel.daterangepicker': function(ev, ev1) {
          $scope.datePicker.date = {};
          // $scope.setRange();
          var picker = $element.find('#daterange4').data('daterangepicker');
          picker.setStartDate(new Date);
          picker.setEndDate(new Date);
        }
      }
  </code></pre>
  
  <p>That clears both date. In my case I want to have option to clear any one date.</p>