Add script to easily run these linter configs on external directories

[rogerluan]

Use case:

1. I fork/clone a project that isn't mine 🍴
2. My OCD distracts me from reading the project because its code style is not consistent 🥵
3. I run something like swiftlinters /path/to/this/new/fork 🧑‍💻
4. Now I can work in peace, more productive, without bikeshedding on code style 😇