Add Validator::postValidate(func)

This method allows passing a function to run when all the filters
and postFitlers have been applied. This is useful to perform
additional validation checks after the regular validation has
been applied.

The function is passed [fields, validOnly].

If validOnly is true, the function must delete any invalid values
from the fields object. No return value is necessary. The fields
object is mutated a seen fit by the validation function.

If validOnly is false AND there are any validation errors, the
function must return an error message.

Example code:

validator.postValidate(function(fields, validOnly) {
  if (fields.password !== fields.password_confirm) {
    if (validOnly) {
      delete fields.password;
      delete fields.password_confirm;
    } else {
      return "Passwords don't match";
    }
  }
});

Multiple postValidation functions can be added, which will then
run in order when performing the post validation.