code: func (config *Config) GetString(key string, defaultValue string) (string, error) {
  	value, err := config.Get(key, defaultValue)
  	if err != nil {
  		return defaultValue, err
  	}
  	if result, ok := value.(string); ok {
  		return result, nil
  	}
  	return defaultValue, &ValueTypeError{key: key, value: value, message: "Value is not a string."}
  }
docs: // GetString fetches the string value by indicating the key.
  // It returns a ValueTypeError if the value is not a sring.