code: func (b Base) URL(styles ...string) string {
  	if b.Url != "" && len(styles) > 0 {
  		ext := path.Ext(b.Url)
  		return fmt.Sprintf("%v.%v%v", strings.TrimSuffix(b.Url, ext), styles[0], ext)
  	}
  	return b.Url
  }
docs: // URL return file's url with given style