- Electron version: v0.36.9
- Operating system: Windows 10/OS X

In #779 a `disposition` was added to the `new-window` event in [webContents](https://github.com/atom/electron/blob/master/docs/api/web-contents.md#event-new-window) but it always seems to be set to `new-window` when clicking on target="_blank" links in our web app wrapper: https://github.com/irccloud/irccloud-desktop/blob/v0.1.3/app/main.js#L109

If I Ctrl (Win) or Cmd (OS X) or middle click on links, I would expect to see a `background-tab` disposition, as in Chrome that will open links in a background tab, while keeping focus on the current tab. I'd like to catch this behaviour to set `activate` to false in `shell.openExternal`.

Is this possible?

atom/browser/api/atom_api_app.cc
*prevent_default = Emit("open-file", file_path);
}
void App::OnOpenURL(const std::string& url) {
  Emit("open-url", url);
}
void App::OnActivate(bool has_visible_windows) {
  Emit("activate", has_visible_windows);
}
void App::OnWillFinishLaunching() {
  Emit("will-finish-launching");
}
void App::OnFinishLaunching() {
  Emit("ready");
}
void App::OnLogin(LoginHandler* login_handler) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default = Emit(
      "login",
      WebContents::CreateFrom(isolate(), login_handler->GetWebContents()),
      login_handler->request(),
      login_handler->auth_info(),
      base::Bind(&PassLoginInformation, make_scoped_refptr(login_handler)));
  
  if (!prevent_default)
    login_handler->CancelAuth();
}
void App::AllowCertificateError(