`\ldots{}.` doesn't behave as well as `\ldots.` with the latex
ellipsis package. This patch causes pandoc to avoid emitting
the `{}` when it is not necessary. Now `\ldots` and other
control sequences used in escaping will be followed by either
a `{}`, a space, or nothing, depending on context.
Thanks to Elliott Slaughter for the suggestion.
This now allows raw LaTeX environments, `\ref`, and `\eqref` to
be parsed (which is helpful for translation HTML documents using
MathJaX).
Closes#1126.