Add support for customizable date format (daattali#536)

Author: bangjunyoung

index.html

@@ -19,7 +19,7 @@ <h3 class="post-subtitle">
     </a>
 
     <p class="post-meta">
-      Posted on {{ post.date | date: "%B %-d, %Y" }}
+      Posted on {{ post.date | date: site.date_format }}
     </p>
 
     <div class="post-entry-container">