FEATURE: add option to add yaml document start (---) to the files output by `tk export`

[lukejolly-simspace]

While writing tooling around yaml files generated with tanka export, I find myself regularly wishing that the docs started with --- and having to script around the fact that they do not. It would be nice if this was an option exposed in a flag. This is a feature I would be willing to add if there is support from the maintainers.

[zerok]

Sounds good to me 🙂 I wonder if we could even not put that behind a flag but, I guess, some folks are already scripting their way around it which would lead to empty documents in the merged stream 😐