Automatically disable rules with lots of violations in --gen-config #617

--gen-config produces a list of excludes to disable a rule in all the files that violate it.
When a rule is violated in many files, this list grows so large that it's not very practical to use.

For example, running ameba --gen-config on https://github.com/crystal-lang/crystal found 584 violations of the Lint/NotNil rule in 151 different files. There's not much point in listing that many files. The exact filenames are largely noise (cf. #566) and provide no meaningful information.

To reduce the weight of the generated config, we can entirely disable a rule when the excludes list would be very long

Rubocop does this as well: https://docs.rubocop.org/rubocop/configuration.html#automatically-generated-configuration
If the number of excluded files is more then 15, it replaces the exclude list with an Enabled: false. The limit is configurable with the --exclude-limit option. It can be disabled with --no-exclude-limit.
I suggest implementing the same behaviour with the same options in ameba.

Maybe --no-exclude-limit is unnecessary and could be expressed as --exclude-limit=0.

Previous discussion in #566