Allow Per-Field Config (value, valid, invalid, etc)

[dominicbarnes]

It would be great if I could define configuration (e.g. value, valid and invalid callbacks) for individual fields.

This would be especially helpful when mixing vanilla form elements with things like yields/select or other highly interactive components. (that don't expose a .value() method)

[ianstormtaylor]

nice, yeah im down. i think it would just involve wrapping the current field adapter setting in clone and then exposing the same functions on the fields to as is on the form. happy to merge it in

[dominicbarnes]

I'll be needing this here at work, so I'll probably send a couple PRs your way tomorrow or next week.