[feature request] Handle recfiles like yaml #2242

Thanks for the excellent tool, John.

Could recutils be treated more like YAML, with 'sub-values'?

Extending sample2.rec:

%rec: Book
%doc: A book in my personal collection.

Title: GNU Emacs Manual
Author: Richard M. Stallman
Topic: Ediff
Topic: ERC
Topic: ERT

Converting this to YAML, the expected result would be:

    $ mlr --irecutils --oyaml cat sample2.rec

- Title: GNU Emacs Manual
  Author: Richard M. Stallman
  Topic:
    - Ediff
    - ERC
    - ERT

But the actual result looks like the output of rec2csv:

  - Title: GNU Emacs Manual
    Author: Richard M. Stallman
    Topic: Ediff
    Topic_2: ERC
    Topic_3: ERT

Miller can sort-of parse it with rename already:

	$ mlr --irecutils --oyaml \
	rename -r '"Topic$",Topic_1'\
	then rename -r '"_",.' \
	then cat book.rec

...but this does not feel tidy, and requires stating the field's name up-front.

Fully rendered Recfiles would be great, as it's the only plain-text and git-friendly relational database.