- 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.h
static mate::Handle<App> Create(v8::Isolate* isolate);
 protected:
  App();
  virtual ~App();
  
  void OnBeforeQuit(bool* prevent_default) override;
  void OnWillQuit(bool* prevent_default) override;
  void OnWindowAllClosed() override;
  void OnQuit() override;
  void OnOpenFile(bool* prevent_default, const std::string& file_path) override;
  void OnOpenURL(const std::string& url) override;
  void OnActivate(bool has_visible_windows) override;
  void OnWillFinishLaunching() override;
  void OnFinishLaunching() override;
  void OnLogin(LoginHandler* login_handler) override;
  
  void AllowCertificateError(
      content::WebContents* web_contents,
      int cert_error,
      const net::SSLInfo& ssl_info,
      const GURL& request_url,
      content::ResourceType resource_type,
      bool overridable,
      bool strict_enforcement,
      bool expired_previous_decision,
      const base::Callback<void(bool)>& callback,
      content::CertificateRequestResultType* request) override;
  void SelectClientCertificate(
      content::WebContents* web_contents,
      net::SSLCertRequestInfo* cert_request_info,