Consider adopting tableschema pattern metadata

[JDziurlaj]

There are several patterns that have developed around tableschema use, but aren't formally part of the spec. The most important one in my view is the addition of metadata to the tableschema itself. Right now we are storing versioning information in the datapackage, but if #29 is implemented, it would have to go somewhere else. We can add the same metadata as in the datapackage using this unofficial vocabulary.
The rationale for doing this would be to better work with tooling that expects a tableschema. For example, data-curator creates its own datapackage from the metadata entered in its interface, but we would still like to know which version of the tableschema was used to validate the feed, as it will influence downstream transformations (i.e. upgrading the feed to be ingested).
(I'd also note that data-curator doesn't write out this unofficial metadata either, so we'd have to fork/PR that change)

[JDziurlaj]

See also qcif/data-curator#1003