Allow files to be excluded

Would it be possible for `elm-format` to allow files to be excluded, e.g. based on some glob or something? The use-case is for when you want to run `elm-format --validate` on the folder containing all your source, but there are some generated files in there that you want to exclude. Examples of generated files are files generated by [elm-graphql](https://github.com/dillonkearns/elm-graphql) or [postcss-elm-tailwind](https://github.com/monty5811/postcss-elm-tailwind). The reason for wanting to exclude them is because they are generated (so you have no control over whether they do/don't conform to the `elm-format` rules), and/or because they are very large and can slow down the validation process.

The issue names no implementation files or tests. Start by tracing the elm-format --validate command path and its handling of a source folder, then determine how an exclusion pattern should interact with generated files; done means excluded files are skipped while other Elm files are still validated.

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