[Fix #988] Add --exclude-limit option

This option can be used with `--auto-gen-config` to set the maximum number of `Exclude` properties to generate per cop. The normal behavior to generate `Enabled: false`, etc, is
kept by default and when the number of files with offenses exceeds the given limit.

I think the new option will be useful for some people. It shouldn't be used always, and the added functionality has to be weighed against the added complexity.