External Validators

I been using your library for a few months now and I've quite liked it. However, I came across a circumstance where I needed external (functional) validation. I couldn't figure out how to do this with the library as it was (if there's a way, I must have missed the documentation).

This is an extremely bare-bones implementation:
  A selector that is checked against each validated field to determine whether validation is run
  A validation function that takes the value of the field and returns a Boolean result