source/_includes/custom/asides/flattr.html

_config.yml

default_asides: [asides/recent_posts.html, asides/twitter.html, asides/github.html, asides/googleplus.html, custom/asides/flattr.html]
# ... snip ...
# Flattr
flattr_user: mogr
flattr_blog_thing: http://flattr.com/thing/582242/Not-a-blog

flattr.html

{% if site.flattr_blog_thing %}
<a class="FlattrButton" style="display:none;" rev="flattr;button:compact;" href="{{ site.url }}"></a>
<noscript>
  <a href="{{ site.flattr_blog_thing }}" target="_blank">
    <img src="http://api.flattr.com/button/flattr-badge-large.png" alt="Flattr this" title="Flattr this" border="0" />
  </a>
</noscript>
{% endif %}