Add CLI flag for non-default label numbering

[anderslundstedt]

For example, in a CV one wants item blocks at level 0 numbered by arabic numerals instead of by latin letters:

(1) My super-influential paper on rock-paper-scissors.

(2) My Nobel-prize-winning paper.

(3) One of my many AI-written papers.

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[ericjohannesson]

Instead of a flag, one could perhaps override the default item numbering in parts of a document by using custom labels like '[1]', '[a]' and '[i]', mutatis mutandis for display lines?

[anderslundstedt]

I prefer the CLI flag option. It simplifies the grammar very much without making the semantics much more complex. Also one might for some reason use ’1’, ’i’ and ’a’ as custom labels.

Going the CLI route does of course not allow for overriding parts of a document. I see this as a very niche use case though, that we should not try to support until an actual need arises?