revealjs (development version)

Add toc and toc_depth argument in revealjs_presentation() as with rmarkdown::html_document(). Set toc = TRUE to create a slide after the title slide containing a Table Of Content. toc_depth = 3 by default (as Pandoc) - its value must be adjusted with slide_level is you do not get the desired result, depending if you are using 2D slides or not.
smart argument has been removed from revealjs_presentation() has no more used by rmarkdown. smart = TRUE is still the default. If you need to deactivate, use md_extensions = "-smart".
Add title-slide id on the auto generated title slide to ease the styling using CSS.
Support more Pandoc's variable in revealjs_presentation() as documented in Pandoc for HTML slides:

background-image to customize the option parallaxBackgroundImage. This duplicates the variable parallaxBackgroundImage which still get precedence.
institute to provide another line in title slide between author and date
toc-title to provide a Title above the toc when toc = TRUE

Fix template to add the necessary CSS to format columns layout and other Pandoc's features (thanks, @atusy, #82).
Fix an issue with some configurations keys for chalkboard plugin - multiple value are now correctly passed to Pandoc's template (thanks, @grayknight2, @atusy, #62, #88).
Fix issues with quoting of options when passed as variables to Pandoc. This was caused by a change in Pandoc 2.14.0.3 which now sets some default value for reveal.js options (thanks, @iain-palmer, #72).
Add md_extensions argument in revealjs_presentation() (thanks, @atusy, #75).

revealjs 0.9