Add extensions.vcl and the type_pipeline extension points

[kptdobe]

For adobe/helix-cli#812 and as discussed there, we need:

• add a extensions.vcl file
• the file will contain 2 empty subroutines:
  • hlx_type_pipeline_before
  • hlx_type_pipeline_after
• the extensions.vcl is included by helix.vcl
• the 2 new subroutines are called at the beginning and the end of hlx_type_pipeline subroutine
• the extensions.vcl file is upload to Fastly exactly like the other vcl files

The override capability is handled in #79

[kptdobe]

While writing the code, I think extensions.vcl makes more sense than custom.vcl.