Added 'paper' after $papersize$ variable in latex template.

Thus you can say `papersize: a4` and the latex will contain
`a4paper`.

This change may break some existing workflows; if you currently
specify `a4paper`, you'll get `a4paperpaper` which is meaningless.

However, the change seems worth it, as it will make the
`papersize` variable work uniformly across ConTeXt, LaTeX,
and html->pdf via wkhtmltopdf.

default.latex

@@ -1,4 +1,4 @@
-\documentclass[$if(fontsize)$$fontsize$,$endif$$if(lang)$$babel-lang$,$endif$$if(papersize)$$papersize$,$endif$$for(classoption)$$classoption$$sep$,$endfor$]{$documentclass$}
+\documentclass[$if(fontsize)$$fontsize$,$endif$$if(lang)$$babel-lang$,$endif$$if(papersize)$$papersize$paper,$endif$$for(classoption)$$classoption$$sep$,$endfor$]{$documentclass$}
 $if(fontfamily)$
 \usepackage[$for(fontfamilyoptions)$$fontfamilyoptions$$sep$,$endfor$]{$fontfamily$}
 $else$