EPUB2 dc:date metadata parsing

[danielweck]

https://github.com/readium/architecture/blob/master/streamer/parser/metadata.md#publication-date

The string is the value of the <dc:date> element whose opf:event attribute has the value publication.

In the Feedbooks OPDS feed I see many publications with:

    <dc:date opf:event="original-publication">1925</dc:date>
    <dc:date opf:event="ops-publication">2011-02-13</dc:date>

JSON schema: https://github.com/readium/webpub-manifest/blob/ee2491413fe33133c42569c61e9aa1a51deb0d52/schema/metadata.schema.json#L32-L46