Support for kebab case

[matthewvelie]

I would love to see support for kebab case. I wrote a simple version here:

 func kebabCased(_ format: StringCaseFormat.SnakeCase = .lower) -> String {
        let split = self.caseSplit()
        if format == .lower {
            return split.map { $0.lowercased() }.joined(separator: "-")
        } else if format == .upper {
            return split.map { $0.uppercased() }.joined(separator: "-")
        }
        return split.map { $0.capitalized }.joined(separator: "-")
    }