Preserve link IDs when possible

[Beanow]

As a small readability feature when looking at the output markdown, it'd be nice if existing IDs in docs like:

[Have you a link][some-link-id]

[some-link-id]: https://github.com/msrd0/cargo-doc2readme

Were preserved and didn't turn into:

[Have you a link][__link0]

[__link0]: https://github.com/msrd0/cargo-doc2readme

It looks like this would depend on https://github.com/raphlinus/pulldown-cmark/issues/434 Which got resolved, but is not yet released in a new crate version (seems planned to be v0.10.0). https://github.com/raphlinus/pulldown-cmark/blob/80eeca9ca87b55e5114db019b93b50aadc2a66cf/src/lib.rs#L187-L188

[msrd0]

It looks like this is blocked on a new release from pulldown-cmark.