- 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_web_contents.h
v8::Local<v8::Value> GetOwnerBrowserWindow();
  
  v8::Local<v8::Value> Session(v8::Isolate* isolate);
  content::WebContents* HostWebContents();
  v8::Local<v8::Value> DevToolsWebContents(v8::Isolate* isolate);
  v8::Local<v8::Value> Debugger(v8::Isolate* isolate);
  
  static void BuildPrototype(v8::Isolate* isolate,
                             v8::Local<v8::ObjectTemplate> prototype);
 protected:
  explicit WebContents(content::WebContents* web_contents);
  WebContents(v8::Isolate* isolate, const mate::Dictionary& options);
  ~WebContents();
  
  bool AddMessageToConsole(content::WebContents* source,
                           int32_t level,
                           const base::string16& message,
                           int32_t line_no,
                           const base::string16& source_id) override;
  bool ShouldCreateWebContents(
      content::WebContents* web_contents,
      int32_t route_id,
      int32_t main_frame_route_id,
      int32_t main_frame_widget_route_id,
      WindowContainerType window_container_type,
      const std::string& frame_name,
      const GURL& target_url,
      const std::string& partition_id,
      content::SessionStorageNamespace* session_storage_namespace) override;
  content::WebContents* OpenURLFromTab(