Added Codable conformity to DrawItem

[blu3mo]

Issue #21
I've added Codable conformity to DrawItem.

encode(to encoder:) first converts CGMutablePath to UIBezierPath, which conforms to NSSecureCoding.
It then converts UIBezierPath to Data using NSKeyedArchiver.
Further Information: https://www.hackingwithswift.com/example-code/language/how-to-store-nscoding-data-using-codable

DrawItem could be encoded/decoded by the code below.

let encodedData = JSONEncoder().encode(drawItem)
let decodedDrawItem = JSONDecoder().decode(SwiftyDrawView.DrawItem.self, from: encodedData)

[blu3mo]

The code below could be used to test encoding and decoding.

    var encodedDatas: [Data] = []
    
    @IBAction func encodeDrawItems() {
        encodedDatas = []
        for drawItem in drawView.drawItems {
            let encodedData = try! JSONEncoder().encode(drawItem)
            encodedDatas.append(encodedData)
        }
        
        drawView.clear()
    }
    
    @IBAction func decodeData() {
        drawView.clear()
        
        for encodedData in encodedDatas {
            let decodedDrawItem = try! JSONDecoder().decode(SwiftyDrawView.DrawItem.self, from: encodedData)
            drawView.addLine(decodedDrawItem)
        }
        
        drawView.setNeedsDisplay()
    }

[LinusGeffarth]

Thanks for your contribution!