Fixed breakpoints, Lazy load iframes

Author: jgthms

_includes/menu.html

@@ -86,10 +86,10 @@
     <strong>{{site.subtitle}}</strong> <small>Created by <a href="https://twitter.com/jgthms">@jgthms</a></small>
   </p>
   <div class="footer-facebook">
-    <iframe src="https://www.facebook.com/plugins/like.php?href=https%3A%2F%2Fwww.facebook.com%2Fcssreference%2F&width=100&layout=button_count&action=like&size=small&show_faces=false&share=false&height=20&appId=1212191075486670" width="100" height="20" scrolling="no" frameborder="0" allowTransparency="true"></iframe>
+    <iframe class="footer-iframe" data-src="https://www.facebook.com/plugins/like.php?href=https%3A%2F%2Fwww.facebook.com%2Fcssreference%2F&width=100&layout=button_count&action=like&size=small&show_faces=false&share=false&height=20&appId=1212191075486670" width="100" height="20" scrolling="no" frameborder="0" allowTransparency="true"></iframe>
   </div>
   <div class="footer-github">
-    <iframe src="https://ghbtns.com/github-btn.html?user=jgthms&repo=css-reference&type=star&count=true" frameborder="0" scrolling="0" width="100px" height="20px"></iframe>
+    <iframe class="footer-iframe" data-src="https://ghbtns.com/github-btn.html?user=jgthms&repo=css-reference&type=star&count=true" frameborder="0" scrolling="0" width="100px" height="20px"></iframe>
   </div>
   <!-- <p class="footer-share footer-share--contribute">
     <strong class="footer-share-label">Contribute</strong>