convert the delegate design to support closure

[av-feaster]

delegate doesn't make the function flexible . Should I get the callback when the gif is loaded or couldn't be loaded using the escaping closure just like sd_webimage
`

   public enum GifEvent {
    case didStart
    case didLoop
    case didStop
    case didFinish
    case didFail(URL, Error?)
}

public struct GifCallbackData {
    public let sender: UIImageView
    public let event: GifEvent
}

public typealias GifCallback = (GifCallbackData) -> Void

  func setGifFromURL(_ url: URL,
                      manager: SwiftyGifManager = .defaultManager,
                      loopCount: Int = -1,
                      levelOfIntegrity: GifLevelOfIntegrity = .default,
                      session: URLSession = URLSession.shared,
                      showLoader: Bool = true,
                      customLoader: UIView? = nil,
callback:@escaping GifCallback = {_ in }) -> URLSessionDataTask?

`

[alexiscreuzot]

not sure about converting it but we could have it as another usage option. Could you do a PR?

[abbassabeti]

@alexiscreuzot Please have a look at This PR.