TOC for Markdown should now work for both Pandoc < 2.0 and >= 2.0

The use of $toc$ and $table-of-contents$ flags changed, see:
https://pandoc.org/releases.html#pandoc-2.0-29-oct-2017
under "Behaviour changes"

bin/template.html

@@ -54,7 +54,13 @@ $endif$
 $endif$
 $if(toc)$
 <nav id="$idprefix$TOC">
-$table-of-contents$
+$if(table-of-contents)$
+  <!-- pandoc >= 2.0 -->
+  $table-of-contents$
+$else$
+  <!-- pandoc < 2.0 -->
+  $toc$
+$endif$
 </nav>
 $endif$
 $body$