latex: Use \subtitle command for subtitle, and give fallback defn.

The fallback definition is a no-op.  In a class that does not
define `\subtitle`, the `subtitle` will not appear.

This changes earlier behavior, which was to add the subtitle
to the title.  This had bad results with page headers etc.

default.latex

@@ -166,7 +166,11 @@ $if(dir)$
 $endif$
 
 $if(title)$
-\title{$title$$if(subtitle)$\\\vspace{0.5em}{\large $subtitle$}$endif$}
+\title{$title$}
+$endif$
+$if(subtitle)$
+\providecommand{\subtitle}[1]{}
+\subtitle{$subtitle$}
 $endif$
 $if(author)$
 \author{$for(author)$$author$$sep$ \and $endfor$}