add a send invoice to alternative e-mail address field to contact
use case: the payer is not the direct customer. We send the invoice for a service that a person requested to the e-mail address of the insurance of that person. To send this invoice from Books we have to create a secondary contact person with the e-mail address of the insurance as of now. We think this could be simplified having a field inside the contact that can be used when the invoice should be sent via e-mail to a different address.