Add option to disableEventMessageParsing and receive messages as Strings

[lavenj]

Hi @nuclearace! Not sure where I should direct this, but I had a need to parse my own messages on a version two(?) websocket that doesn't support binary data, and added a new config option:

/// If passed `true`, event message data will not be parsed, and all message events will be received with
/// `event` value of `"rawMessage"`; listen with `socketClient.on("rawMessage") { ... }`
case disableEventMessageParsing(Bool)

If enabled, event messages are received like so:

socketClient.on("rawMessage") { data, ack in //data is [String] ... }

And emit() calls don't do any further parsing of their data argument, and are expected to look something like:

let message:String = """[echo, {"text": "someValue"}]""" // looks like: [<messageName>, <payload>]"
socketClient.emit("rawMessage", message) //first parameter is ignored

I'm sure there's a more beautiful way of implementing this, but I wanted to keep changes minimal.

Upside for me is easy parsing from/to strongly typed objects using Codable

Any interest in merging?

Example usage:

//receiving the rawMessage
socketClient.on("rawMessage") { data, ack in
	guard let s = data.first as? String, let data = s.data(using: .utf16) else { return }
	let decoder = JSONDecoder()
	let message = try! decoder.decode(Message.self, from: data)
	//do whatever!
}

Parsing/Decoding:

enum Message: Decodable {
	case yellowAlert(YellowAlert)
	case redAlert(RedAlert)

	enum MessageType: String, Decodable {
		case yellowAlert
		case redAlert
	}

	init(from decoder: Decoder) throws {
		var values = try decoder.unkeyedContainer()
		let type = try values.decode(MessageType.self)
		switch type {
		case .yellowAlert:
			let payload = try values.decode(YellowAlert.self)
			self = .yellowAlert(payload)
		case .redAlert:
			let payload = try values.decode(RedAlert.self)
			self = .redAlert(payload)
		}
	}
	
	//Empty data types for demo purposes. You'd probably have some properties in real data.
	struct YellowAlert: Decodable { }
	struct RedAlert: Decodable { }
}