Pushing stanza buliding logic down into stanzas.

This is an intermediate step, since I think Stanzas now contains logic
about two things: the stanzas of the rhyme and how to construct a
stanza.
Extraction of a Stanza

Here we extract out the stanza logic from the Stanzas. This is better.

I still find this hard to extend. A requirement change for a new stanza
or a reordering of stanzas would require opening up existing code. But
absent those requirements, I'm not sure there's much more to do here.
Extraction and refactoring

Preparing (possibly) for requirement changes be extracting out the
linking_article and preamble so that they can be injected. And I chose
to consolidate all of my puts/prints into one method, so that I only
have one place to change when I start to refactor out the presentation