Support Decodable/Encodable (Swift 4) about filekit HOT 3 CLOSED

These would be good changes to have but behind #if swift(>=3.2).

from filekit.

I make a class FileKit to get shared JSON/PropertyList encoder/decoder
And also for easy to use functions

try FileKit.write(encodable, to: .userTemporary + "test")
let decodable: MyClass = try FileKit.read(from:  .userTemporary + "test")

Array of Codable are codable, so the same functions work

I add protocols JSONReadable, JSONWritable (or PropertyListReadable/PropertyListWritable) to be able to implement Readable and Writable and create File with specific type just by implementing protocols
(an alias name could be added like JSONReadableWritable if necessary for JSONWritable, JSONReadable)
This protocol define the encoder or decoder to use. By default the one shared by FileKit

struct TestJSONCodable: Codable {
    let string: String
    let integer: Int
}
extension TestJSONCodable: JSONWritable, JSONReadable {}

let encodable = TestJSONCodable(string: "test", integer: 9)
let codableFile = File<TestJSONCodable>(path:  .userTemporary + "testcodable.json")
try codableFile.write(encodable)
let decodable: TestJSONCodable = try codableFile.read()

from filekit.

committed but need some doc in README

from filekit.