Add a way to tell the linter to ignore specified directories

[zachary-foster]

Description of feature

Is there a way to configure the linter to ignore certain files/directories? As part of how our pipeline works, it creates very large caches of downloaded data to be reused. These typically end up in the source directory during testing. Linter checks like the one that looks for unhanded merge conflicts never finish on these 500Gb+ folders. I have to move these directories out manually to run the linting and then move them back in for testing.

@ewels suggested this could be implemented by ignoring any files in .gitignore. I imagine this would work well in most cases, at least as a good default.