added header to customizable includes

.themes/classic/source/_includes/custom/header.html

@@ -0,0 +1,6 @@
+<hgroup>
+  <h1><a href="{{ root_url }}/">{{ site.title }}</a></h1>
+  {% if site.subtitle %}
+    <h2>{{ site.subtitle }}</h2>
+  {% endif %}
+</hgroup>

.themes/classic/source/_includes/header.html

@@ -1,6 +1 @@
-<hgroup>
-  <h1><a href="{{ root_url }}/">{{ site.title }}</a></h1>
-  {% if site.subtitle %}
-    <h2>{{ site.subtitle }}</h2>
-  {% endif %}
-</hgroup>
+{% include custom/header.html %}