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.


The other `user_context*.vcl`s have it too. It’s because they are being used directly in the functional tests. In the other functional tests, `fos.vcl` is used (which has `vcl 4.0` again), but then any other files are included, so they don’t require the header.