Cancel replace on pjax:beforeReplace

[jspizziri]

I would like to suggest being able to cancel pjax on the beforeReplace event.

    //Check if replace should be canceled
    if(!fire('pjax:beforeReplace', [container.contents, options], {
      state: pjax.state,
      previousState: previousState
    })){
        return false;
    }

Here is my use case: If a user is logged in and their session times out the next pjax call will load the result of the unauthenticated request (likely a login form). In this instance I would like to be able to cancel pjax gracefully so that I may present a login UI that I desire.

[jspizziri]

Pull Request: #428

[mislav]

Closing in favor of your PR