Rewrite in Swift + Support SwiftPM

Let's rewrite this library in Swift ~~3.0~~ :tada:

This is a **long term goal**, and mostly for fun. Especially with nullability annotations, there is very little value to gain by doing this.

But, Swift is awesome™ and the future™, so it would be fun to rewrite this. Also, it's a pretty small framework so it shouldn't be too difficult or take too long.

~~:warning: Waiting for the final release of Swift 3.0 to do this.~~

Todos:
- [ ] Rewrite in Swift
- [ ] Update docs
- [ ] Update unit tests
- [ ] Update travis-ci
- [ ] Rewrite demo projects in Swift

Some notes:
- Use an `enum FileType` for the file types (file extensions)
- Use a `struct` for file names, which contains the name (`String`) and type (`FileType`)
- [Singletons in Swift](https://thatthinginswift.com/singletons/)

> ### ⚠️ NOTE: will not accept pull requests for this issue :smile:

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Any new features we can make use of in this library?

#### 5.1
- https://www.hackingwithswift.com/articles/182/whats-new-in-swift-5-1
- https://www.raywenderlich.com/4187396-what-s-new-in-swift-5-1

#### 5.2
- https://swift.org/blog/swift-5-2-released/
- https://www.hackingwithswift.com/articles/212/whats-new-in-swift-5-2