index.md: add tagline metadata

Instead of hard coding the tagline use some metadata

Author: philips

_includes/themes/twitter/page.html

@@ -1,5 +1,5 @@
 <div class="page-header">
-  <h1>{{ page.title }} <small>Supporting tagline</small></h1>
+  <h1>{{ page.title }} <small>{{ page.tagline }}</small></h1>
 </div>
 
 <div class="row">

index.md

@@ -1,6 +1,7 @@
 ---
 layout: page
 title: Hello World!
+tagline: Supporting tagline
 ---
 {% include JB/setup %}