Add vcl_deliver extension point

[kptdobe]

In order to control the headers just before the response gets delivered, we would need a hlx_vlc_deliver_after extension point. This would help solving adobe/helix-pages#17.
To respect the current pattern, I would also add the hlx_vlc_deliver_before extension point, even though I do not think it is useful for now.

[kptdobe]

Just found out that vcl_deliver might not be the right place... Fastly still caches the result of the pipeline somewhere.