Add option to ignore files

Please add a configuration opportunity to exclude particular files from the linter.

Either allow globbing for the config.files option:

files: ["**/[a-cA-C]*.md"]

Or add a new configuration option, explicitly excluding files (again, with globbin):

exclude: ["**/__*.md"]

[elgalu]

Also setting --ignore in a pre-commit works like a charm, e.g.

  - repo: https://github.com/igorshubovych/markdownlint-cli
    rev: "596386e6a7da92547c4fc6b94fce59bcbc8290ee" # frozen: v0.27.1
    hooks:
      - id: markdownlint
        args: ["--ignore", "towncrier-template.md"]

[SetTrend]

Works like a charm. Thanks!