More control for post excerpts

Changed excerpt behaviour. Posts can have individually defined
excerpts. No more truncation after 50 words. “Read More” is only
displayed if there is more content.

index.html

@@ -22,8 +22,16 @@ <h3 class="post-subtitle">
     </p>
 
     <div class="post-entry">
-      {{ post.content | strip_html | xml_escape | truncatewords: 50 }}
-	  <a href="{{ post.url | prepend: site.baseurl }}" class="post-read-more">[Read&nbsp;More]</a>
+      {{ post.excerpt }}
+      {% capture content_words %}
+        {{ post.content | number_of_words }}
+      {% endcapture %}
+      {% capture excerpt_words %}
+        {{ post.excerpt | number_of_words }}
+      {% endcapture %}
+      {% if excerpt_words != content_words %}
+        <p><a href="{{ post.url | prepend: site.baseurl }}" class="post-read-more">[Read&nbsp;More]</a></p>
+      {% endif %}
     </div>
 
    </article>