Add support for a baseline file

[shatodj]

As the developer, I want to add new rules to the .swiftformat, but I don't want to be reported or autocorrected in my entire codebase, just the new files (lines) added to the project.

• Add a command that will create a baseline report file (JSON) to have a list of the issues (lint errors, possibly)
• Add a command to be able to skip the reported changes in the baseline file

[nicklockwood]

Yeah, this is something I've wanted to add for literally years. It should be simple enough as we already have the cache to determine which files have changed