salsa_get_supporter adds supporter if not found

It took me a while to figure this one out (after creating many salsa records with just an email address).

Perhaps a new function could be created called salsa_get_supporter_add_on_failure() that provides this functionality, so that salsa_get_supporter could simply works as is intuitive.

Or, a new argument could be added that would indicate whether a record should be added on failure.