wishlist: expand $committer etc in revision_mail_headers

Hi,

bzr-email expands things like $committer in the message subject and body. I think it would be useful if it also expanded them in the values of any revision_mail_headers setting. This enables me to do things like:

revision_mail_headers = "Mail-Followup-To: <email address hidden>, $committer"

It seems easy to implement:

***************
*** 188,194 ****
              return
          for line in headers:
              key, value = line.split(": ", 1)
! result[key] = value
          return result

      def send(self):
--- 188,194 ----
              return
          for line in headers:
              key, value = line.split(": ", 1)
! result[key] = self._format(value)
          return result

      def send(self):