Toggle table of contents via front matter (#1310)

* Add jekyll-toc include
* Reduce whitespace generated by comments
* Add table of contents include to `single` layout
* Replace `toc` include with jekyll-toc enabled YAML Front Matter
* Update README
* Update table of contents documentation
  - Revise `toc` helper include to mention that it will be deprecated in the next major version.
  - Add documentation to `single` layout explaining how to enable table of contents on those pages.
* Update CHANGELOG and history
* Update LICENSE

Close #1222

_layouts/single.html

@@ -34,6 +34,14 @@ layout: default
       {% endunless %}
 
       <section class="page__content" itemprop="text">
+        {% if page.toc %}
+          <aside class="sidebar__right">
+            <nav class="toc">
+              <header><h4 class="nav__title"><i class="fa fa-{{ page.toc_icon | default: 'file-text' }}"></i> {{ page.toc_label | default: site.data.ui-text[site.locale].toc_label }}</h4></header>
+              {% include toc.html sanitize=true html=content h_min=2 h_max=3 class="toc__menu" %}
+            </nav>
+          </aside>
+        {% endif %}
         {{ content }}
         {% if page.link %}<div><a href="{{ page.link }}" class="btn btn--primary">{{ site.data.ui-text[site.locale].ext_link_label | default: "Direct Link" }}</a></div>{% endif %}
       </section>