See: https://github.com/readium/architecture/blob/master/streamer/parser/metadata.md#identifier https://github.com/readium/r2-shared-js/blob/a01efc9bcae240449365ad23741c3c056a87b604/src/parser/epub.ts#L1144-L1156 https://github.com/readium/r2-shared-js/blob/a01efc9bcae240449365ad23741c3c056a87b604/src/parser/epub/opf-identifier.ts#L28-L29 https://github.com/readium/r2-shared-js/blob/a01efc9bcae240449365ad23741c3c056a87b604/src/parser/epub/opf-metadata.ts#L36-L38 https://github.com/readium/r2-shared-js/blob/a01efc9bcae240449365ad23741c3c056a87b604/src/parser/epub/opf-metafield.ts#L31-L38 Related issue: https://github.com/readium/r2-streamer-swift/issues/113
