groff expects this and treats . and ? differently when
followed by line ending as opposed to ordinary space.
Also, don't escape periods. Instead, use zero-width character
\& to avoid unwanted interpretation of periods at start of line.
Resolves Issue #148.
git-svn-id: https://pandoc.googlecode.com/svn/trunk@1580 788f1e2b-df1e-0410-8736-df70ead52e1b