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

**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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