Add exclude option to cli and configuration toml

[thetawom]

PR adds an exclude option to the CLI (and to the configuration TOML) for excluding files matching the specified patterns. This provides a way to run mdformat outside of pre-commit but still with the ability to exclude certain files.

Resolves #359

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[ringohoffman]

@hukkin @chrisjsewell any updates on this? I would really like to see this feature included.

[twmht]

This really helps! thank you @thetawom