Currently, when a user calls `makeSingleInstance` they need to handle the second instance in a callback.

A better pattern for both users and third-party modules who want to use this information would be to emit an event on the `app` module when the second instance starts.

A pattern like:

```js
app.makeSingleInstance();

app.on('second-instance', (e, argv) => {
  // do-stuff
});
```

As step 1 I'll introduce support for the event and deprecate the callback.  Then in a major version, we can remove the callback.
atom/browser/api/atom_api_app.cc
scoped_refptr<net::X509Certificate> cert,
                   scoped_refptr<net::SSLPrivateKey> private_key) {
  delegate->ContinueWithCertificate(cert, private_key);
}
void OnClientCertificateSelected(
    v8::Isolate* isolate,
    std::shared_ptr<content::ClientCertificateDelegate> delegate,
    std::shared_ptr<net::ClientCertIdentityList> identities,
    mate::Arguments* args) {
  if (args->Length() == 2) {
    delegate->ContinueWithCertificate(nullptr, nullptr);
    return;
  }
  v8::Local<v8::Value> val;
  args->GetNext(&val);
  if (val->IsNull()) {
    delegate->ContinueWithCertificate(nullptr, nullptr);
    return;
  }
  mate::Dictionary cert_data;
  if (!mate::ConvertFromV8(isolate, val, &cert_data)) {