make metadata header optional

[therealglazou]

edasfr thinks metadata should be optional, the only mandatory thing being the html title element

Source: https://n.survol.fr/n/e0-le-retour-du-fils-de-la-vengeance

[frivoal]

See also #10 for how to figure out the document title if we allow dc:title to be dropped along with the <header>.