Add site.feed.updated == latest_post setting

Primary uses cases:

1. Reduce deployment churn in repositories that hold a docsite in
   addition to source code. These repositories currently regenerate
   and find "changed" files to deploy on every commit, even when
   nothing in the Jekyll site was changed. The only artefact changing
   each time is `feed.xml`.

   For example: https://github.com/qunitjs/qunit/commits/gh-pages

2. Improve reproducibility of the build, as highlighted via
   jekyll/jekyll#7187. By offering a choice
   that simply eliminates use of current time entirely, we do not
   even have to worry about mocking it.

README.md

@@ -45,6 +45,17 @@ feed:
 
 To note, you shouldn't have to do this unless you already have a feed you're using, and you can't or wish not to redirect existing subscribers.
 
+### Updated timestamp
+
+By default, the feed will set `<updated>` to the current time at build time, as provided by Jekyll via `site.time`.
+
+Enable the `updated: latest_post` setting to use a deterministic value based on when the latest post was published or updated. (The [jekyll-last-modified-at](https://github.com/gjtorikian/jekyll-last-modified-at) plugin is supported to help provide post modification times).
+
+```yml
+feed:
+  updated: latest_post
+```
+
 ### Optional front matter
 
 The plugin will use the following post metadata, automatically generated by Jekyll, which you can override via a post's YAML front matter:

lib/jekyll-feed/feed.xml

@@ -6,7 +6,6 @@
   <generator uri="https://jekyllrb.com/" version="{{ jekyll.version }}">Jekyll</generator>
   <link href="{{ page.url | absolute_url }}" rel="self" type="application/atom+xml" />
   <link href="{{ '/' | absolute_url }}" rel="alternate" type="text/html" {% if site.lang %}hreflang="{{ site.lang }}" {% endif %}/>
-  <updated>{{ site.time | date_to_xmlschema }}</updated>
   <id>{{ page.url | absolute_url | xml_escape }}</id>
 
   {% assign title = site.title | default: site.name %}
@@ -51,7 +50,20 @@
     {% assign posts = posts | where_exp: "post", "post.draft != true" %}
   {% endunless %}
   {% assign posts = posts | sort: "date" | reverse %}
+
   {% assign posts_limit = site.feed.posts_limit | default: 10 %}
+
+  {% assign last_updated = nil %}
+  {% if site.feed.updated == "latest_post" %}
+    {% for post in posts limit: posts_limit %}
+      {% assign post_updated = post.last_modified_at | default: post.date %}
+      {% if last_updated == nil or post_updated > last_updated %}
+        {% assign last_updated = post_updated %}
+      {% endif %}
+    {% endfor %}
+  {% endif %}
+  <updated>{{ last_updated | default: site.time | date_to_xmlschema }}</updated>
+
   {% for post in posts limit: posts_limit %}
     <entry{% if post.lang %}{{" "}}xml:lang="{{ post.lang }}"{% endif %}>
       {% assign post_title = post.title | smartify | strip_html | normalize_whitespace | xml_escape %}

spec/jekyll-feed_spec.rb

@@ -118,6 +118,7 @@
 
   context "parsing" do
     let(:feed) { RSS::Parser.parse(contents) }
+    let(:overrides) { { "time" => Time.parse("2018-12-31") } }
 
     it "outputs an RSS feed" do
       expect(feed.feed_type).to eql("atom")
@@ -136,6 +137,24 @@
       expect(feed.generator.version).to eql(Jekyll::VERSION)
     end
 
+    context "with site.feed.updated = latest_post" do
+      let(:overrides) do
+        {
+          "feed" => {
+            "updated" => "latest_post",
+          },
+        }
+      end
+
+      it "outputs the last post time" do
+        expect(feed.updated.content).to eql(Time.parse("2016-04-25"))
+      end
+    end
+
+    it "outputs the current time" do
+      expect(feed.updated.content).to eql(Time.parse("2018-12-31"))
+    end
+
     it "includes the items" do
       expect(feed.items.count).to eql(10)
     end