Linting should fail when baseline violations are fixed

[swizzlr]

New Issue Checklist

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

Feature or Enhancement Proposal

As with other linter baseline implementations, a fix to a violation that was previously baselined should cause a lint failure that encourages the user to update the baseline. For backwards compatibility, this should probably be a flag, e.g. --strict-baseline or something (feel free to bikeshed).

[swizzlr]

If there's an appetite for this I'll contribute.

[SimplyDanny]

No objections. You may go ahead.