Override rules from `parent_config`

I have the config:

parent_config: https://github.com/sindresorhus/swiftlint-config/raw/723a1fa/.swiftlint.yml

In the parent config, I use only_rules to define the rules I want. I want to reuse the https://github.com/sindresorhus/swiftlint-config/raw/723a1fa/.swiftlint.yml config in many projects, but in some projects, I might want to add some extra rules, and maybe disable some. This is currently not possible. I think only_rules in a child config should be able to extend a only_rules in a parent config. This is possible in any other linter, like ESLint in the JavaScript ecosystem.

parent_config: https://github.com/sindresorhus/swiftlint-config/raw/723a1fa/.swiftlint.yml
only_rules:
  - attributes
disabled_rules:
  - todo

Results in:

SwiftLint Configuration Error: 'disabled_rules' or 'opt_in_rules' cannot be used in combination with 'only_rules' – Falling back to default configuration

// @fredpi

Start by tracing SwiftLint's configuration handling for parent_config, only_rules, and disabled_rules. Reproduce the reported configuration error, then verify that child rules can extend or disable rules from the parent configuration without falling back to the default configuration.