Print leading '---'

[msiebuhr]

It would make it trivial to apply multiple yamlfmt'ed files for sending over pipes and such:

cat lots-of-yaml/* | ssh bastion ... kubectl apply -f -

(ATM, I have to apply them one a time OR manually stitch them together).

While I've mostly been hand-writing YAML (horay for auto-formatting!), quite a few linters get annoyed about the missing trible-dash in the beginning of the document.