Eat blank lines around \item and \begin/\end{enumerate}/{itemize}
Eat blank lines around \item and \begin/\end{enumerate}/{itemize}

This fixes some weirdness in Markdown where blank lines break paragraphs
in the middle of lists, e.g.

```
\begin{enumerate}
\item hi

\item bye
\end{enumerate}
```

edemaine/coauthor,edemaine/coauthor