Quoter

Extended Markdown feature with header function ``h``

Start of a Markdown capability. It is at a experimental, proof of
concept, or demonstration stage rather than production-use stage
at present--much as HTML and XML quoting functions were a few
releases back. The initial goal is to stretch the ``quoter`` use-
case and prove/harden its extension mechanisms. Even at its
inaugural stage, it is doing just that.

Reorg of XML and HTML code into own module. Parallel reorg of
tests. All tests passing.

Starts automatic collection of test branch coverage. Begins with
99% coverage.

Enables cloning and specialization of all ``Quoter`` subclasses,
including ``xml`` and ``html`` objects docs previously warned were
non-functional. Refactors majority of ``Joiner`` functionality as
a subclass of ``Quoter``, with all the rights and privileges
thereunto appertaining.

Extends tests.

Added "naked" invocations of primary quoter front-ends, such as
``html("joe", "b.one")``. This allows quoters to take on primary
information (such as the tag name) during each call. This was
long planned for the API, but had been sidelined due to now-
quashed complexity.