Added most important pieces of metadata for publication

This fixes what's more important about #106.
See [Returning metadata about the document](https://github.com/w3c/specberus/blob/tripu/issue-106-metadata/README.md#returning-metadata-about-the-document) for more information about the way metadata is returned.
Y U NO enforcing digit constraint? :)

This actually makes me think of a use case I didn't think of before: if a document specifies to an editor ID that doesn't exist (or is not part of the WG which is trying to publish, but we actually do not even check that on the current system), it will pass Specberus and subsequent checkers, the process will pursue with TR installation and publication... and fail when calling the publication system! I also realize that solving this is not trivial at all without adding a new checker and interaction with the W3C DB!