add citation, prism, and dc metadata

[inhumantsar]

this adds support for metadata fields common to journals and other academics-adjacent sites.

breaking changes:

• byline is now an array of strings if multiple authors are detected (see ietf-1 and nature)

other changes:

• JSON-LD parsing will be retried with the mainEntity field will be parsed if no relevant metadata is found on the first try
• _normalizeByline will attempt to clean up the byline field. initially, i was going to try and swap Surname, GivenName formatting to GivenName Surname here as well, but that was going to be problematic when multiple authors are listed within a single property (as in videos-2).
• generate-testcase.js will now attempt to follow redirects. this mirrors the behaviour of the library and was required to generate the nature test case.
• target version for eslintrc was bumped from ES6 to ES2017 to allow generate-testcase.js's use of async and =>.

[AramZS]

dc: is Dublin Core and it is very widely used in academic circles. I think this would be a great addition!