Make 'updateModel' supporting Promise?

Hi guys,
Thanks for this great lib, it really helps a lot. I got one question about 'updateModel' that for this one is really suitable for some validation hook, to support server side validations, could we make it support something like promise?

Thanks

[akre54]

Sure, wanna open a pull?

@akre54 , Thanks, will do it later this week.

PR #305