Escape post.title in atom.xml

If post.title were to contain one of `&`, `<`, or `>`, the XML would become invalid. `xml_escape` is a [Jekyll extension of Liquid](https://github.com/mojombo/jekyll/blob/master/lib/jekyll/filters.rb#L59-L72) to safely escape values.

Using straight Liquid, this could also be done with:

```html
<title type="html">
    {{ post.title | escape
                  | replace:'&','&amp;'
                  | replace:'<','&lt;'
                  | replace:'>','&gt;' }}
</title>
```

(Note, `type=html` and then the [additional XML escaping](http://atomenabled.org/developers/syndication/#text).)