Fix invalid XML in feed

`post.content` contains HTML. Truncating it arbitrarily at 400 chars
could result in raw `&` characters for example to end up in the feed.

This commit simply strips HTML and then truncates.

Fixes quarkusio#213

feed.xml

@@ -17,7 +17,7 @@ layout: null
                 {{ post.url | prepend: site.url }}
             </link>
             <description>
-                {{ post.content | escape | truncate: '400' }}
+                {{ post.content | strip_html | truncate: '400'  }}
             </description>
             <pubDate>{{ post.date | date_to_rfc822 }}</pubDate>
             <guid>