custom_action()
  Interscanner to set custom action.
  
  This scanner can be used to set custom action (reject, drop, deliver)
  by manually specifing this as parameters.
  You can use these variables in format string:
    %(VIRNAME)s         name of detected virus, empty if CLEAN
    %(QNAME)s           quarantine file name (quarantine must be inside
                          custom_action scanner to use this)
    %(SCANNER_NAME)s    scanner which reported this virus
    %(VERSION)s         sagator's version
  
  Usage: custom_action(pattern, reply_code, reply_message, scanners)
  
  Where: pattern is an regular expression, which defines virus/spam to
           set this action. It is checked over virus name.
         reply_code is an integer or string, which defines message reply
           code (250 for success, 451 for temporary failure, 550 for
           reject).
         reply_message is an string, which defines reply message string.
           You can use variables defined above here.
  
  Bugs: Does not working with milter() service.
  
  Example: custom_action(".", 550, "Content rejected - %(VIRNAME)s",
             scanners...
           )
  
  New in version 0.9.0.