Revert "remove history option"

This reverts commit cddd0ae6a88e9f35b2c45f4f0cea9042c53eb627.
diff --git a/R/revealjs_presentation.R b/R/revealjs_presentation.R
index d8ea2e6..29628df 100644
--- a/R/revealjs_presentation.R
+++ b/R/revealjs_presentation.R
@@ -84,6 +84,7 @@
                                   theme = "simple",
                                   transition = "default",
                                   background_transition = "default",
+                                  history = TRUE,
                                   highlight = "default",
                                   mathjax = "default",
                                   template = "default",
@@ -141,6 +142,8 @@
   background_transition <- match.arg(background_transition, revealjs_transitions())
   args <- c(args, "--variable", paste("backgroundTransition=", background_transition, sep=""))
   
+  # history
+  args <- c(args, pandoc_variable_arg("history", jsbool(history)))
   
   # content includes
   args <- c(args, includes_to_pandoc_args(includes))