Allow keyboard navigation in hack tutorial

Fixes facebook/hhvm#4879

js/hack_tutorial/tutorial.js

@@ -17,6 +17,7 @@ var remove_comments = function (content) {
 
 (function () {
     var current = 0,
+        codeset = {17: false, 66: false, 78: false}, // ctrl, b, n
         passed_test = function(content) { return true; },
         prev_button = document.getElementById('prev_button'),
         next_button = document.getElementById('next_button'),
@@ -58,6 +59,32 @@ var remove_comments = function (content) {
           resetState();
         });
 
+        // navigating with keys: ctrl-b for back, ctrl-n for next
+        window.addEventListener('keydown', function(e) {
+          if (e.keyCode in codeset) {
+            codeset[e.keyCode] = true;
+            if (codeset[17] && codeset[66]) {
+              current = (current <= 0) ? 0 : current - 1;
+              resetState();
+            } else if (codeset[17] && codeset[78]) {
+              if (current >= ft_exercises.length - 1) {
+                current = ft_exercises.length;
+                editor.setValue("// Congratulations! You are done!");
+              } else {
+                current++;
+              }
+              resetState();
+            }
+          }
+        });
+
+        window.addEventListener('keyup', function(e) {
+          if (e.keyCode in codeset) {
+            codeset[e.keyCode] = false;
+          }
+        });
+
+
     editor.setValue(ft_exercises[current].content);
     resetState();
 

js/hack_tutorial/tutorial_source.js

@@ -8,6 +8,8 @@ ft_exercises = Array(
 \n\
 \n\
 /************ Click 'Next' to get started! *********/\n\
+\n\
+/************ You can use buttons OR `ctrl-b` and `ctrl-n` to navigate *********/\n\
 ", check:(function (x) { var content = remove_comments(x);return (true);})}, {content:"<?php\n\
 // ^-- FIXME: replace <?php with <?hh\n\
 \n\