title: Add a class to react-day-picker?
body: <p>How can I add a class to react-day-picker's today button? </p>
  
  <p>It seems to be possible from the documentation:
  <a href="http://react-day-picker.js.org/api/DayPicker#classNames" rel="nofollow noreferrer">http://react-day-picker.js.org/api/DayPicker#classNames</a></p>
  
  <pre><code>const dayPickerClassNames = { todayButton: 'newClass' };
  
  &lt;DayPicker
    classNames={dayPickerClassNames}
  /&gt;
  </code></pre>
  
  <p>However Im getting an error:</p>
  
  <pre><code>Warning: Failed prop type: The prop `classNames.day` is marked as required in `DayPicker`, but its value is `undefined`.
  </code></pre>