SwiftPM support

• Migrate to Swift 5 and Xcode 10.2 & 11.2 (#20, kudos to @olejnjak)
• Add public inits to built-in loggers so they can really be used (#21, kudos to @olejnjak)
• Add SPM support (#23, kudos to @olejnjak)

SPM release also supports macOS 🎉