Possible types definition in the validator

Hi, I was expecting something like this to be already possible in the validator? I disliked defining my own model validator to do this, so I extended the validator.

I am not sure this is the way you imagined it, so I just leave it up to you to use if you like.

I am sorry, no tests, I was not able to get the dummy app specs to run. Would appreciate if you could add a line in Readme on how to run tests.

Thanks for the great gem, real life saver!