Add includes for static-based comments powered by Staticman

- Configure Staticman with sane defaults
- Build comment form to capture new comments and ajax data to Staticman
- Build comments view that iterates over `_data/comments/post-slug/*.yml` files

_includes/comment.html

@@ -0,0 +1,22 @@
+<article id="comment{{ include.index }}" class="js-comment comment">
+  <div class="comment__avatar-wrapper">
+    <img class="comment__avatar" src="https://www.gravatar.com/avatar/{{ include.email }}?d=mm&s=80">
+  </div>
+  <div class="comment__content-wrapper">
+    <h3 class="comment__author">
+      {% if (include.url) and (include.url != '@url') %}
+        <a rel="nofollow" href="{{ include.url }}">{{ include.name }}</a>
+      {% else %}
+        {{ include.name }}
+      {% endif %}
+    </h3>
+    <p class="comment__date">
+      {% if include.date %}
+        {% if include.index %}<a href="#comment{{ include.index }}">{% endif %}
+        <time datetime="{{ include.date | date_to_xmlschema }}">{{ include.date | date: "%B %d, %Y at %I:%M %p" }}</time>
+        {% if include.index %}</a>{% endif %}
+      {% endif %}
+    </p>
+    {{ include.message | markdownify }}
+  </div>
+</article>