Don't let math span over multiple paragraphs
Don't let math span over multiple paragraphs

Fix #203.  Experiments in LaTeX show that it forbids paragraph breaks
(via multiple newlines or \par) in all forms of math: $...$, $$...$$,
\begin{equation} ... \end{equation}, \begin{align} ... \end{align}, etc.
Close math mode whenever we encounter such a paragraph break.

edemaine/coauthor,edemaine/coauthor