It is now possible for blog-posts to link to next/prev.

This closes #17.

The template `entry.tmpl` is loaded with the contents of a
single post, and now if there are some details of the next/previous
post for each entry.  This allows:

      <table width="100%"><tr>
         <td valign="top" width="50%" align="left"><p><!-- tmpl_if name='prev_link' --><a href="<!-- tmpl_var name='prev_link' -->"><!-- tmpl_var name='prev_title' --></a><!-- /tmpl_if name='prev_link' --></p> </td>
         <td valign="top" width="50%" align="right"><p><!-- tmpl_if name='next_link' --><a href="<!-- tmpl_var name='next_link' -->"><!-- tmpl_var name='next_title' --></a><!-- /tmpl_if name='next_link' --></p></td>
      </tr></table>

i.e. `next_link`, `next_title`, `prev_link` and `prev_title` are
all populated.