Implement custom format extension regexps (or move to own repo?)

[dmitrizagidulin]

Implement the custom format extensions used in the schemas:

• StringOrURI
• NumericDate
• URI
• url
• base64
• base64url
• MediaType

Better yet, I think they should be moved upstream to json-document and defined there (since they're likely to be very useful outside of jose).

Also, relace usages of BASE64_REGEXP with base64url.

[christiansmith]

I think most of them need to stay here. We should only implement the ones in JSON Schema that are defined there.

Also, now that we have npm orgs for open source, will probably break json-document up into several packages like we're doing with jose. That package is due for some major attention in the near future, probably right after we wrap up other commitments for 9/30.