code: func AddHandler(h Handler, levels ...Level) {
  	rw.Lock()
  	for _, level := range levels {
  		handler := append(logHandlers[level], h)
  		logHandlers[level] = handler
  	}
  	rw.Unlock()
  }
docs: // AddHandler adds a new log handler and accepts which log levels that
  // handler will be triggered for