Implement MathJax

If a post needs to render math equations, it only has
to include the mathjax.json params.

layout/post.html

@@ -3,6 +3,10 @@
         <article>
             <h1>{{ title }}</h1>
 
+            <p>// Posted {{ date }}</p>
+
+            {{ mathjax_warning }}
+
             {{ content }}
         
             <br />