- 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/atom_browser_client.cc
content::WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    scoped_ptr<content::ClientCertificateDelegate> delegate) {
  
  auto cmd = base::CommandLine::ForCurrentProcess();
  if (cmd->HasSwitch(switches::kClientCertificate)) {
    auto cert_path = cmd->GetSwitchValuePath(switches::kClientCertificate);
    auto certificate = ImportCertFromFile(cert_path);
    if (certificate.get())
      delegate->ContinueWithCertificate(certificate.get());
    return;
  }
  if (!cert_request_info->client_certs.empty() && delegate_) {
    delegate_->SelectClientCertificate(
        web_contents, cert_request_info, std::move(delegate));
  }
}
void AtomBrowserClient::ResourceDispatcherHostCreated() {
  resource_dispatcher_host_delegate_.reset(
      new AtomResourceDispatcherHostDelegate);
  content::ResourceDispatcherHost::Get()->SetDelegate(
      resource_dispatcher_host_delegate_.get());
}
brightray::BrowserMainParts* AtomBrowserClient::OverrideCreateBrowserMainParts(
    const content::MainFunctionParams&) {
  v8::V8::Initialize();  
  return new AtomBrowserMainParts;
}
void AtomBrowserClient::WebNotificationAllowed(