LaTeX template: add variable romanfont (#4665)

This opens up the possibility of setting the romanfont in xelatex/lualatex.
Note that mainfont actually oversets sansfont in headings

data/templates/default.latex

@@ -92,6 +92,9 @@ $endfor$
 $if(mainfont)$
     \setmainfont[$for(mainfontoptions)$$mainfontoptions$$sep$,$endfor$]{$mainfont$}
 $endif$
+$if(romanfont)$
+	\setromanfont[$for(romanfontoptions)$$romanfontoptions$$sep$,$endfor$]{$romanfont$}
+$endif$
 $if(sansfont)$
     \setsansfont[$for(sansfontoptions)$$sansfontoptions$$sep$,$endfor$]{$sansfont$}
 $endif$