Add "pay what you can" option for conference registration

Is your feature request related to a problem? Please describe.
We often want our conferences to allow participants to "pay what you can", entering an amount of their choice to register.

Describe the solution you'd like
Allow the administrator to select "Pay what you can" for any class of registration, after which the "price" would become a minimum amount for that class of registraion.

Describe alternatives you've considered
We have to do offboard registration because of this limitation, on an external Drupal site.