code: func StreamHandler(wr io.Writer, fmtr LogFormat) LogHandler {
  	h := FuncHandler(func(r *Record) error {
  		_, err := wr.Write(fmtr.Format(r))
  		return err
  	})
  	return LazyHandler(SyncHandler(h))
  }
docs: // StreamHandler writes log records to an io.Writer
  // with the given format. StreamHandler can be used
  // to easily begin writing log records to other
  // outputs.
  //
  // StreamHandler wraps itself with LazyHandler and SyncHandler
  // to evaluate Lazy objects and perform safe concurrent writes.