Add key bindings for up/down arrows (prev/next)

index.html

@@ -78,6 +78,10 @@
       markdown: {
         smartypants: true,
       },
+      keyboard: {
+        38: 'prev',  // Up arrow
+        40: 'next'   // Down arrow
+      },
       math: {
 		mathjax: 'reveal.js/lib/js/mathjax/MathJax.js',
 		config: 'TeX-AMS_HTML-full'  // See http://docs.mathjax.org/en/latest/config-files.html