Open a WKWebview target="_blank" link in Safari

First add WKNavigationDelegate and webviewWk.navigationDelegate = self

func webView(webView: WKWebView, decidePolicyForNavigationAction navigationAction: WKNavigationAction, decisionHandler: (WKNavigationActionPolicy) -> Void) {

        //this is a 'new window action' (aka target="_blank") > open this URL externally. If we´re doing nothing here, WKWebView will also just do nothing. Maybe this will change in a later stage of the iOS 8 Beta
        if navigationAction.navigationType == WKNavigationType.LinkActivated {
            println("here link Activated!!!")
            let url = navigationAction.request.URL
            let shared = UIApplication.sharedApplication()

            let urlString = url!.absoluteString

            if shared.canOpenURL(url!) {
                shared.openURL(url!)
            }

            decisionHandler(WKNavigationActionPolicy.Cancel)
        }

        decisionHandler(WKNavigationActionPolicy.Allow)
    }

In (from here)

 override func loadView() {
    super.loadView()
    self.webView.navigationDelegate = self 
    self.webView.UIDelegate = self  //must have this
 }

Then add the function (from here, with additions)...

func webView(webView: WKWebView,
    createWebViewWithConfiguration configuration: WKWebViewConfiguration,
    forNavigationAction navigationAction: WKNavigationAction,
    windowFeatures: WKWindowFeatures) -> WKWebView? {
        if navigationAction.targetFrame == nil {
            var url = navigationAction.request.URL
            if url.description.lowercaseString.rangeOfString("http://") != nil || url.description.lowercaseString.rangeOfString("https://") != nil || url.description.lowercaseString.rangeOfString("mailto:") != nil  {
                UIApplication.sharedApplication().openURL(url)
            }
        }
        return nil
}

Code updated for iOS 10 Swift 3:

override func loadView() {
    super.loadView()
    self.webView.navigationDelegate = self 
    self.webView.uiDelegate = self  //must have this
}

func webView(_ webView: WKWebView,
               createWebViewWith configuration: WKWebViewConfiguration,
               for navigationAction: WKNavigationAction,
               windowFeatures: WKWindowFeatures) -> WKWebView? {
    if navigationAction.targetFrame == nil, let url = navigationAction.request.url {
      if url.description.lowercased().range(of: "http://") != nil ||
        url.description.lowercased().range(of: "https://") != nil ||
        url.description.lowercased().range(of: "mailto:") != nil {
        UIApplication.shared.openURL(url)
      }
    }
  return nil
}