Setting to have SwiftLint exclude files listed in `.gitignore` or other existing ignore files

[rgoldberg]

New Issue Checklist

• I've Updated SwiftLint to the latest version.
• I've searched for existing GitHub issues.

Feature or Enhancement Proposal

Please add a setting to have SwiftLint exclude files listed in .gitignore or in other existing ignore files.

I want to lint all Swift files in my repo, but .build contains many generated Swift files that get linted.

.build & other paths are already ignored by my .gitignore; I shouldn't need to copy such ignored paths into .swiftlint.yml.

Other file names or ignore file formats can also be supported. You could always take the union of all ignores from all supported ignore files from standard locations, allow format & path to be specified, and allow multiple ignore files, potentially with multiple ignore files per same type.