Handle relative and suffix references

One can use quasiquotations to specify relative URI refs and suffix
URI refs.  See RFC 3986 §4.2 and §4.5 for details.

There's one piece of trickery here.  I'm not yet sure if I like it.  I
wanted a localized way for quasiquotations to specify a base URI
against which relative URIs can be resolved.  I considered Prolog
flags and a special configuration ref.  Both options require global
state and pushing/popping that state surrounding a predicate
definition.

I realized that local variables have exactly the local reach that I
wanted.  I further realized that quasiquotations expansion has access
to all local variables.  By assigning a base URI to __uri_qq_base, one
changes the way that relative URI references are resolved.  It works
great, but it's a little strange since nothing else in Prolog behaves
this way.
Write documentation
Ignore some generated files
Fill out pack file omissions
Point to the GitHub page