Provide hlx_owner, hlx_repo and hlx_ref extension points

[kptdobe]

With #85, I thought that hlx_type_pipeline would be enough but that not the case since hlx_type_pipeline is using X-Owner, X-Repo and X-Ref to compute other stuff. The todo here is:

• create hlx_owner_before and hlx_owner_after empty subroutines in extensions.vcl
• create hlx_repo_before and hlx_repo_after empty subroutines in extensions.vcl
• create hlx_ref_before and hlx_ref_after empty subroutines in extensions.vcl
• call the hlx_*_before and hlx_*_after subroutines in the corresponding hlx_* subroutines.