Add an option to link previous and next posts in post metadata

[jmglov]

If you set the :link-prev-next-posts option to true, quickblog adds prev
and next metadata to each post (where prev is the previous post and next
is the next post in date order, oldest to newest). You can make use of these by
adding something similar to this to your post.html template:

{% if any prev next %}
  <div class="post-prev-next">
{% if prev %}
    <div>⏪ <a href="{{prev.file|replace:.md:.html}}">{{prev.title}}</a></div>
{% endif %}
{% if next %}
    <div><a href="{{next.file|replace:.md:.html}}">{{next.title}}</a> ⏩</div>
{% endif %}
  </div>
{% endif %}

• This PR corresponds to an [issue with a clear problem statement] Link posts #90
• I have updated the CHANGELOG.md file with a description of the addressed issue.

[borkdude]

Thanks a lot