Fix #466 next and previous links

A fix relating to #466.  Next and previous links are added to the article object.  These can be accessed in a template like so:

    {% if article.next_url %}
    <a href="{{article.next_url}}">Next</a>
    {% endif %}

and 

    {% if article.previous_url %}
    <a href="{{article.previous_url}}">Previous</a>
    {% endif %}


I had to add the `previous_url` and `next_url` to the default metadata in the generator... this wasn't required to get the templates working but it was required for the tests to pass. Have I missed something obvious here?