Swift Fourtification

Regex now speaks Swift 4! 🎉

Added:

• The Regex type now conforms to Codable, so you can encode and decode regular expressions to your heart's content. (#52)
• MatchResult has a new range property. (#54, thanks @mezhevikin!)
• To go with MatchResult.range, there's also the captureRanges property, allowing you to work with the ranges of capture groups. (#59)

Fixed:

• Regex now handles the newly overhauled String index API, maintaining that sweet, sweet Unicode correctness, whatever your chosen Swift version. (#57, 7952bae, 1a90514, thanks @wspl!)

Thanks for using Regex!