Support for Swift Package Manager

[janlionly]

Swift Package Manager

• iOS: Open Xcode, File->Swift Packages, search input https://github.com/radex/SwiftyTimer.git, and then select Version Up to Next Major 2.1.1 < .
• Or add dependencies in your Package.swift:

.package(url: "https://github.com/radex/SwiftyTimer.git", .upToNextMajor(from: "2.1.1")),

Please git a new tag to support it.