make vspace above passage header configurable and increase default
to 4pt.
make poetry formatting parameters configurable.
make paragraph struct, and prepare for accumulating footnotes
and cross-references in their own paragraph structures.
initialise all paragraphs on startup.
begin overhauling footnote system. allow two character footnote
marks for a total of 26+676 footntoes per page.
render each footnote into separate footnote paragraphs.
clear out footnote paragraphs when reseting footnote state
work on footnotes and cross-reference handling.
allow typesetting to complete with dummy reenumerate_footnotes()
record the paragraph line on which a footnote occurs to support
reenumeration of footnote marks when paragraphs are split over
multiple pages.