Explicitly enable syntax highlighting

Author: erickt

presentations/intro-to-rust/index.html

@@ -1136,7 +1136,10 @@
     <script src="remark.js" type="text/javascript">
     </script>
     <script type="text/javascript">
-      var slideshow = remark.create();
+      var slideshow = remark.create({
+        highlightStyle: 'default',
+        highlightLines: true,
+      });
     </script>
   </body>
 </html>