Implement external browser launch in Quicksling CEF

[jasonjei]

We need to be able to launch an external browser from the CEF browser. Apparently, the suggested way to do this is intercept the link click in OnBeforeNavigate and use OS functions to launch the URL, perhaps by including a special tag attribute.

Here's a SO article that documents how to do it:
http://stackoverflow.com/questions/31241240/how-to-open-a-link-in-a-native-browser-from-cefsharp-3