code: func newDefaultFrontmatterConfig() frontmatterConfig {
  	return frontmatterConfig{
  		date:        []string{fmDate, fmPubDate, fmLastmod},
  		lastmod:     []string{fmGitAuthorDate, fmLastmod, fmDate, fmPubDate},
  		publishDate: []string{fmPubDate, fmDate},
  		expiryDate:  []string{fmExpiryDate},
  	}
  }
docs: // This is the config you get when doing nothing.