Add start of document "---" in the beginning of outputed YAML document

[damienleger]

Describe the problem/challenge you have
YAML output produced by YTT don't include the start of document --- in the beginning (first document).

Assuming I generate file-1.yml and file-2.yml with YTT, I'm used to do things like cat file-1.yml file-2.yml | kubectl apply -f - but since file-2.yml doesn't start with --- it doesn't work.

Describe the solution you'd like
Either add a flag to YTT to include --- on beginning of every document (even the first one), or make this behavior default.

Anything else you would like to add:
No

---

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[cppforlife]

@damienleger i think this is a reasonable change we can make. just have to decide whether we should do it by default. i think we've done something similar for other carvel tools like kbld.

[pivotaljohn]

Any tooling downstream from ytt ought to accept the optional document start (i.e. ---) without problem. 👍🏻

I think this should be the default behavior and have the option to disable it should it either break something (e.g. a highly-precise test), or if there's some interaction with a future feature (e.g. if we ever decide to output comments).

[pivotaljohn]

Rationale for priority:
This looks valuable. It's something that we'd definitely support a contribution on. It's not something the core maintainers are committing to do, right now.

[cari-lynn]

Discussion from duplicate issue:

And to be clear, we're talking about non-empty documents, yeah?

I ask because I know of a few cases where we rely on skipping empty documents to keep the output sane.

We should not add --- on empty documents.