Support Swift Package Manager for Xcode 11

[KaneCheshire]

👋

With Xcode 11 supporting SPM natively (and really well), Cocoapods will quickly die off, so it's a great idea to start thinking about SPM asap.

I'm not sure how this would work with PrettyTimestamp since it's in ObjC, perhaps there can be an exposed Swift wrapper around it, or better yet the whole thing gets re-written in Swift? Now that we have ABI stability the Swift code should be as future proofed as the ObjC code in terms of not needing to be updated too often.

[jonhocking]

Definitely keen and would be great to rewrite the interface some more. I'll see if I can find some time over the next few weeks once I'm back 👍