Enable use of spacebar for scrolling into book body

assets/javascript/app.js

@@ -41,5 +41,8 @@ require([
 
         // Show progress
         progress.show();
+
+        // Focus on content
+        $(".book-body").focus();
     });
 });

templates/page.html

@@ -5,7 +5,7 @@
 <div class="book with-summary" data-github="{{ githubId }}" data-level="{{ progress.current.level }}">
     {% include "includes/book/header.html" %}
     {% include "includes/book/summary.html" %}
-    <div class="book-body">
+    <div class="book-body" tabindex="-1">
         <div class="page-wrapper">
             <div class="book-progress">
                 <div class="bar">