Allow mutli-doc and go-patch pages in ci.yml

[Improvements]

* Supports pages to ci.yml for better pipeline customizations.  These
  pages can be either overlay-style (spruce) or go-patch style.  These
  extra pages will be merged over the pipeline generated by Genesis.

  Because concourse syntax allows non-homogeneous structure, you can't
  always use spruce operators to overlay the right location (ie adding
  an additional param to a specific task config)  In these cases, you
  can use go-patch.

  Example:
  ```
  ---
  pipeline:
    <... all the normal ci.yml configuration values ...>

  ---
  - type: replace
    path: /jobs/name=<env>/plan/0/do/task=bosh-deploy/config/params/MY_ENV?
    value: some-value

  ```

  Note that the first page must be in overlay style, not go-patch,
  because it must stand as a valid yaml hash on its own.

lib/Genesis/CI/Legacy.pm

@@ -1787,7 +1787,7 @@ EOF
 	close $OUT;
 
 	return run({ onfailure => 'Failed to merge Concourse pipeline definition', stderr => 0 },
-		'spruce', 'merge', '--prune', 'meta', '--prune', 'pipeline', "$dir/guts.yml", $pipeline->{file});
+		'spruce', 'merge', '--multi-doc', '--go-patch', '--prune', 'meta', '--prune', 'pipeline', "$dir/guts.yml", $pipeline->{file});
 }
 
 1;