[WIP] 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.


Either this or we ask devs to run `pip install -r requirements`; `requirements.txt` we need anyway for building on ReadTheDocs.