URN Support

[mtiller-jh]

I just want to point out that RFC 3986 covers both URLs and URNs under the umbrella of URIs. But there is no mention of URNs anywhere in this project. Granted, it is possible to parse a URN as a URI. But the data types still have fields that are URL specific (e.g., host, path) which are URL specific and don't contain any of the terms associated with URNs from RFC 8141 (e.g., NID or NSS).

Also, the isvalid function isn't actually performing the checks from RFC 8141 relating to valid URNs. The result is that invalid URNs (e.g., urn:--name:foo) are considered valid by the isvalid function and valid URNs (e.g., urn:example:weather?=op=map&lat=39.56&lon=-104.85&datetime=1969-07-21T02:56:15Z) are considered invalid by the isvalid function.

In short, I think in order to really claim RFC 3986 compatibility, some work needs to be done to add support for RFC 8141 or the documentation should be updated to reflect that the library is really meant for supporting only URLs.

Thoughts?