Add {{ content }} to home layout (#1775)

* Add `{{ content }}` to `home` layout

Allow the author to add content which will appear above the list of
recent posts. This also allows the `home` layout to be extended.

Add text to index.html in tests to indicate that the content appears in
the correct spot.

* Add paragraph markup

Since this is a HTML file, proper paragraph markup is needed.

_layouts/home.html

@@ -2,6 +2,8 @@
 layout: archive
 ---
 
+{{ content }}
+
 <h3 class="archive__subtitle">{{ site.data.ui-text[site.locale].recent_posts | default: "Recent Posts" }}</h3>
 
 {% for post in paginator.posts %}

test/index.html

@@ -2,3 +2,5 @@
 layout: home
 author_profile: true
 ---
+
+<p>This text should appear above the recent posts.</p>