Get the value of sendmail_path() after the on_sent_message hook.

  Previously we  ran:

   1.  get the sendmail_path.
   2.  Call the on_sent_message hook.
   3.  Send the mail, using the value of sendmail from 1.

  We (obviously) should get the sendmail_path after calling the
 hook to cope with users who change it there.

  Closes #149.