Add template basename to template variables

Usage example : 

```html
<ul class="nav">
  <li {% if basename == 'index' %} class="active"{% endif %}><a href="/">Home</a></li>
  <li {% if basename == 'page1' %} class="active"{% endif %}><a href="page1.html">Page 1</a></li>
  <li {% if basename == 'page2' %} class="active"{% endif %}><a href="page2.html">Page 2</a></li>
</ul>
```