Add optional middleware for removing `Set-Cookie`

* In my case, Clearance middleware was adding `Set-Cookie` to every request
* This occured after `set_cache_control_headers` was called, whether it was a
  `before_filter` or `after_filter`
* Optional middleware looks for `Surrogate-Control` header so it only removes
  `Set-Cookie` for endpoints not being cached by Fastly
Merge pull request #28 from jessieay/master

Add optional middleware for removing `Set-Cookie`