[Enhancement Request] Support for non-string form elements

[grgcombs]

We use a variety of form field controls beyond text fields and text views, and many of the backing values aren't suitably transferable to strings for validation within the framework.

For instance, on a form that accepts a start date-time and an end date-time for a calendar event, we have a pair of text fields each backed by UIDatePickerViews. In this case, we have a need to validate that the selected end date is on or after the selected start date. However, it seems like we have to cheat the validator a bit in order to convert the NSDate back and forth to a localized formatted date string as the validator triggers the various conditions.

I wound up building a | greater than | less than | equal to | group of conditions and validators that are backed by - (NSComparisonResult)compare:(id)other ... the upside is that it'll work for NSNumbers (like via a slider or stepper) too. But i'm still not quite satisfied with my janky hack.

Perhaps it would make sense to provide an option for a block-based validation that's more agnostic about the data types?