add `onRender` option

This event is fired when a day is rendered inside the datepicker. Should return a string. Return 'disabled' to disable the day from being selected.

```javascript
$('#date-end')
    .datetimepicker({
        onRender: function(date) {
            return ev.date.valueOf() < date-start-display.valueOf() ? ' disabled' : '';
        }
    });
```