code: func (c *Config) GetString(key string) (string, error) {
  	val, ok := (*c)[key]
  	if !ok {
  		return "", ErrNoSuchKey
  	}
  	s, ok := val.(string)
  	if !ok {
  		return s, fmt.Errorf("Wrong type for %s (Type:%T Expecting:%T)", key, val, s)
  	}
  	return val.(string), nil
  }
docs: // GetString gets the value associated to a key as string
  // return (string, error)