Option to specify path to configuration file

[pawamoy]

Context

Follow-up of #263. Now that config files are supported, I'd like to be able to specify a different path for the config file. The reason is that I put all tools config files into a config folder (name is not important), and call linting tools from my task runner, which takes care of passing the right path to the config file.

Proposal

Adding a simple option to the CLI like mdformat -f config/mdformat.toml (-f because -c is more similar to --check). Long option could be --config or --config-file.

Tasks and updates

No response

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