TOPIC: Ability to have "fields" expire with Early Bird

It would be great to have a check box option on 'fields' to enable the field to expire with Early Bird so that we can offer incentives to early registrants (eg. "Free t-shirt with early registration" with a "pick you size" field that automatically disappears after the early bird deadline passes).

Should only take a few lines of code in admin (and maybe in admin's xml depending on how the backend config handles this part), one additional field in the corresponding mySQL table, and an if() statement in the front end code to implement.