Replace bespoke feed.xml with jekyll-feed plugin

_config.yml

@@ -150,6 +150,7 @@ gems:
   - jekyll-paginate
   - jekyll-sitemap
   - jekyll-gist
+  - jekyll-feed
 
 # Outputting
 permalink: /:categories/:title/

feed.xml

@@ -1,43 +0,0 @@
----
-sitemap: false
----
-
-<?xml version="1.0" encoding="utf-8"?>
-<feed xmlns="http://www.w3.org/2005/Atom" xml:lang="en">
-<title type="text">{{ site.title }}</title>
-<generator uri="https://github.com/jekyll/jekyll">Jekyll</generator>
-<link rel="self" type="application/atom+xml" href="{{ site.url }}/feed.xml" />
-<link rel="alternate" type="text/html" href="{{ site.url }}" />
-<updated>{{ site.time | date_to_xmlschema }}</updated>
-<id>{{ site.url }}/</id>
-<author>
-  <name>{{ site.owner.name }}</name>
-  <uri>{{ site.url }}/</uri>
-  {% if site.owner.email %}<email>{{ site.owner.email }}</email>{% endif %}
-</author>
-{% for post in site.posts limit:20 %}
-{% if post.author %}
-  {% assign author = site.data.authors[post.author] %}
-{% else %}
-  {% assign author = site.owner %}
-{% endif %}
-<entry>
-  <title type="html"><![CDATA[{{ post.title | cdata_escape }}]]></title>
-  <link rel="alternate" type="text/html" href="{{ site.url }}{{ post.url }}" />
-  <id>{{ site.url }}{{ post.id }}</id>
-  {% if post.modified %}<updated>{{ post.modified | to_xmlschema }}T00:00:00-00:00</updated>
-  <published>{{ post.date | date_to_xmlschema }}</published>
-  {% else %}<published>{{ post.date | date_to_xmlschema }}</published>
-  <updated>{{ post.date | date_to_xmlschema }}</updated>{% endif %}
-  <author>
-    <name>{{ author.name }}</name>
-    <uri>{{ site.url }}</uri>
-    {% if author.email %}<email>{{ author.email }}</email>{% endif %}
-  </author>
-  <content type="html">
-    {{ post.content | xml_escape }}
-    {% include feed-footer.html %}
-  </content>
-</entry>
-{% endfor %}
-</feed>