Makes next and previous posts accessible when post is loaded

 closes #529 and closes #685

 -  Full post entry is available for use dependent on context
 - e.g 1 <a href="{{#post.prev}}{{url}}{{/post.prev}}">{{post.prev.title}}</a>
 - e.g 2 {{#post}}{{#prev}}<a href="{{url}}">{{title}}</a>{{/prev}}{{//post}}