Add support for utterances (#1966)

* Add support for utterances
* Add utterances config documentation
* Update CHANGELOG and history

Close #1909

_includes/comments-providers/scripts.html

@@ -12,6 +12,8 @@
     {% include /comments-providers/staticman.html %}
   {% when "staticman_v2" %}
     {% include /comments-providers/staticman_v2.html %}
+  {% when "utterances" %}
+    {% include /comments-providers/utterances.html %}
   {% when "custom" %}
     {% include /comments-providers/custom.html %}
 {% endcase %}

_includes/comments-providers/utterances.html

@@ -0,0 +1,20 @@
+<script>
+  'use strict';
+
+  (function() {
+    var commentContainer = document.querySelector('#utterances-comments');
+
+    if (!commentContainer) {
+      return;
+    }
+
+    var script = document.createElement('script');
+    script.setAttribute('src', 'https://utteranc.es/client.js');
+    script.setAttribute('repo', '{{ site.repository }}');
+    script.setAttribute('issue-term', 'pathname');
+    script.setAttribute('theme', '{{ site.comments.utterances.theme | default: "github-light" }}');
+    script.setAttribute('crossorigin', 'anonymous');
+
+    commentContainer.appendChild(script);
+  })();
+</script>

_includes/comments.html

@@ -165,6 +165,9 @@
           <!-- End new comment form -->
         {% endif %}
       </section>
+    {% when "utterances" %}
+      <h4 class="page__comments-title">{{ comments_label }}</h4>
+      <section id="utterances-comments"></section>
     {% when "custom" %}
       <section id="custom-comments"></section>
   {% endcase %}