latex template: use providecommand for tightlist.

This avoids a conflict when the memoir class is used.
Closes #94.

default.latex

@@ -123,7 +123,7 @@ $endif$
 \setlength{\parindent}{0pt}
 \setlength{\parskip}{6pt plus 2pt minus 1pt}
 \setlength{\emergencystretch}{3em}  % prevent overfull lines
-\newcommand{\tightlist}{%
+\providecommand{\tightlist}{%
   \setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}
 $if(numbersections)$
 \setcounter{secnumdepth}{5}