Add option to disable lowercasing of Reference links

[clee-ai]

Describe the problem

Some scripts that operate on MD files directly depend on the capitalization of the file contents. Right now, mdformat automatically lowercases reference link definitions, for example:

## [Unreleased]
[Unreleased]: https://github.com

mdformat would change to:

## [Unreleased]
[unreleased]: https://github.com

It would be nice to have an option to disable this lowercasing, so that doing text find/replaces within the file don't have to worry about capitalization.

Link to your repository or website

No response

Steps to reproduce

somefile.MD

## [Unreleased]
[Unreleased]: https://github.com

run mdformat to get:

## [Unreleased]
[unreleased]: https://github.com

The version of Python you're using

No response

Your operating system

No response

Versions of your packages

No response

Additional context

No response

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[hukkin]

Thanks for the issue!

I'm not sure I want this as link labels are case insensitive. Lower casing makes this obvious and the link labels consistent.

What would be your expected formatting for the following Markdown?

[unreleased]

[Unreleased]

[Unreleased]: https://github.com

[clee-ai]

In the case of running mdformat with the default settings, I would expect to lowercase them and keep the current behavior.

In the case of disabling case sensitivity, the formatter won't touch the casing, instead leaving it the same.

[unreleased]

[Unreleased]

[Unreleased]: https://github.com

I know with this setting, the formatter wouldn't be as opinionated; however, from a user perspective I would prefer the default behavior to be strongly opinionated, with the ability to disable certain rules. However, if this goes against the design philosophy of the library, I understand, and instead I can try to fix the offending scripts 😄

[hukkin]

Yeah, I do think it's probably better if you fix the offending scripts. If lowercasing link labels breaks your scripts, then I assume the mixed case example also does so, no matter how we format it. Fixing your scripts should be the more resilient solution.

[clee-ai]

Understandable, my perspective is coming from adhering to specifications that define (perhaps improperly) how the file should be formatted. For example, my usecase was from https://keepachangelog.com/en/1.0.0, where the [Unreleased] tag is uppercased. In addition, the order of the link labels is intentional. My thoughts are, it would be nice to disable some features of mdformat (link label ordering/capitalization) while maintaining a consistent formatting on the rest of the file.

[hukkin]

I see your point. But I don't currently feel like this is a strong enough case to justify adding configuration. I don't want to maintain a configurable formatter, but if you or anyone else wants to make a fork adding all sorts of switches, I think that'd actually be pretty great.