Change how letter closes if not specified

The letter will default to `Yours faithfully,` because opening defaults
to `Dear Sir or Madam`. If opening is specified (assuming this is
because you know the name of the person you are writing to) closing will
default to `Yours Sincerely`. This behaviour can still be overidden by
specifying `closing: ...` in the yaml header

pandoc/templates/template-letter.tex

@@ -286,8 +286,12 @@ $endif$
 $if(closing)$
 \closing{$closing$}
 $else$
+$if(opening)$
+\closing{Yours sincerely,}
+$else$
 \closing{Yours faithfully,}
 $endif$
+$endif$
 $if(encl)$
 \encl{$for(encl)$$encl$$sep$\\$endfor$}
 $endif$