Add "--revealjs-title-content" to flags

First off @jgm, Pandoc is a wonderful creation.  Thanks for all your hard work!

Whenever I have to give a presentation, I turn to Pandoc to generate my slides.  Previously, I used slidy, but Reveal.js has really caught my eye.

I love being able to pull a presentation from Markdown, but I don't like being constrained to a certain format (even if the reasons for this are very, very good), particularly the inability to mix vertical and horizontal content slides with Pandoc + Reveal.js.

For that reason, I've thrown together a new CLI option that allows Pandoc to still generate the content for title slides with level < slideLevel when using the Reveal.js writer.  I've already found it useful in piecing together my presentations, and I hope it can benefit others as well!

Again, thanks for all your work on Pandoc & Co!