Adds the function to take the email recipient from a form field

Adds the new InsertTags {{form::fieldname}} (with "fieldname" replaced by the name of your field)  to the recipient field in the form defenition. The value of that field will be taken as recipient for the email. Works with standard forms and EFG.

IMPORTANT: Protect the field with another module (e.g. protectedselect) against manipulation. If you don't do this spammers can abuse the form to send spam!