Add ability in validation to set a response field.
i.e:  a checkbox that triggers the 'send me an email' functionality

- ability to check for novalue (i.e: cgbf-validate-nodata
   to enforce that the field must not have a value on submission
- ability to check for a valid date value (if value exists)  
- add ability to use a data-cgbf-useremail attribute on an input or select
   to designate that field as the 'users email address'
   display this field somehow in the 'fields' table
   use this field as the 'reply to' address in dispositions that send emails to admins
   i.e:  something like:  $form->get_user_email()  which could return null.
