xml escape the post title

I didn't test this change, just a quick edit because I noted an unescaped ampersand was breaking the RSS-Feed in my browser.

feed.xml

@@ -19,7 +19,7 @@
     <description>{{ site.description }}</description>
     {% for post in site.posts limit:10 %}
       <item>
-        <title>{{ post.title }}</title>
+        <title>{{ post.title | xml_escape }}</title>
         <description>{{ post.content | xml_escape }}</description>
         <pubDate>{{ post.date | date: "%a, %d %b %Y %H:%M:%S %z" }}</pubDate>
         <link>{{ site.url }}{{ post.url }}</link>