Support toc-title in revealjs

[fkohrt]

Among other HTML formats, the HTML5 template includes the content of toc-title as heading:

pandoc/data/templates/default.html5

Lines 54 to 61 in 99e1b67

	$if(toc)$
	<nav id="$idprefix$TOC" role="doc-toc">
	$if(toc-title)$
	<h2 id="$idprefix$toc-title">$toc-title$</h2>
	$endif$
	$table-of-contents$
	</nav>
	$endif$

Is there any reason against doing something similar for revealjs? Otherwise there's only an untitled slide containing the TOC, which doesn't look appealing.

[jgm]

Yes I think we could do the same for revealjs. (If you want to submit a PR?)

[fkohrt]

Done. Wasn't sure about the heading level, but went for h2 as with html5. I also copied the nav, but in a separate commit in case you'd rather not have this in there.