Show Varnish configuration in tabs

Fix #130.

Thanks to @xabbuh we can now show config for different Varnish versions in configuration blocks: http://foshttpcache.readthedocs.org/en/doc-tabs/varnish-configuration.html#purge. I guess we need some custom CSS or Sphinx theme to actually render the `<li>`s as tabs.


btw, it might be better to add it here rather than on line 15, to keep it near the comment explaining it