Add Open Graph meta tags

http://ogp.me/
https://stackoverflow.com/q/6535405

themes/typing/layout/_partial/head.ejs

@@ -21,7 +21,8 @@
   %>
   <title><% if (title){ %><%= title %> | <% } %><%= config.title %></title>
   <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
-
+  <%/* Add Open Graph meta tags for easier sharing on social networking sites */%>
+  <%- open_graph() %>
   <% if (theme.rss){ %>
     <link rel="alternate" href="<%- theme.rss %>" title="<%= config.title %>" type="application/atom+xml">
   <% } %>