More spec information and linkage

[NathanaelA]

This probably needs to be a discussion on design, but it would be nice if the tests linked to the actual specification (or perhaps MDN?) and had the year it was added to the specification.

[foolip]

WPT is for the most part organized as a directory per spec and we have META.yml files that have the spec links.

I've suggested surfacing this in #372. If we use https://github.com/w3c/browser-specs we might be able to get some additional information, but not spec age or MDN links. However, entries in https://github.com/mdn/browser-compat-data link to both specs and MDN, so that could be used to create a many-to-many spec-to-MDN mapping.