Support for references in `date.min()` and `date.max()`?

It seems like Joi doesn't currently support references in `date.min()` and `date.max()`. I'm trying to do something like this:

``` js
Joi.object().keys({
  date_range_start : Joi.date(),
  date_range_end   : Joi.date().min(Joi.ref('date_range_start'))
});
```

Would this be an accepted feature if a pull request was made for it?

Thanks!
