Minimize bandwidth utilization for posts

We don't need to download a picture that we don't show anyway.

This patch simply add a HTML5 picture tag, that loads the blog cover for 
bigger sites and has a inline encoded 1x1 transparent pixel as fallback 
for mobile devices.

_layouts/post.html

@@ -3,7 +3,13 @@ layout: default
 ---
 <article class="post-container post-container--single">
   <header class="post-header">
-    {% if page.cover %}<img class="post-cover" src="{{ site.baseurl }}/{{ page.cover }}" alt="Cover image for this blog post"> {% endif %}
+    {% if page.cover %}
+    <picture class="post-cover">
+     <source srcset="{{ site.baseurl }}/{{ page.cover }}" media="(min-width: 960px)">
+     <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX/TQBcNTh" alt="Cover image for this blog post">
+    </picture>
+
+    {% endif %}
     <div class="post-meta">
       <time datetime="{{ page.date | date: "%Y-%m-%d %H:%M" }}" class="post-meta__date date">{{ page.date | date: "%-d %b %Y" }}</time>
       {% if page.tags.size > 0 %}