add optional intro, outro, title and subtitle for the index page

_includes/post_index.html

@@ -1,7 +1,18 @@
-{% if include.title == blank %}
+
+{% assign title=(include.title | default: site.index.title)%}
+
+{% if title == blank %}
 	<h1>{{ include.posts | size }} blogs since {{ include.posts | sort: 'date' | map: 'date' | compact | first | date: '%Y' }}</h1>
 {% else if %}
-	<h1>{{ include.title }}</h1>
+	<h1>{{ title }}</h1>
+{% endif %}
+
+{% if site.index.intro != blank %}
+	{{ site.index.intro }}
+{% endif %}
+
+{% if site.index.subtitle != blank %}
+	<h2>{{ site.index.subtitle }}</h2>
 {% endif %}
 
 {% assign prev_year = 'first run' %}
@@ -35,3 +46,7 @@
 	{% if current_year != date %}
 	{% endif %}
 {% endfor %}
+
+{% if site.index.outro != blank %}
+	{{ site.index.outro }}
+{% endif %}