renderDjot: omit auto generated attributes and reference

[black-desk]

Close #69

Signed-off-by: black-desk me@black-desk.cn

[jgm]

I think I like this plan, even though it makes the model a bit more complex. It would be good to have some automated tests; it is a big change.

[black-desk]

I will add some tests few days later.

[black-desk]

pandoc.spec.ts seems already contains tests for attrs and references.

[black-desk]

@jgm done.