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
v8::Local<v8::Value> val,
                     JumpListCategory::Type* out) {
    std::string category_type;
    if (!ConvertFromV8(isolate, val, &category_type))
      return false;
    if (category_type == "tasks")
      *out = JumpListCategory::Type::TASKS;
    else if (category_type == "frequent")
      *out = JumpListCategory::Type::FREQUENT;
    else if (category_type == "recent")
      *out = JumpListCategory::Type::RECENT;
    else if (category_type == "custom")
      *out = JumpListCategory::Type::CUSTOM;
    else
      return false;
    return true;
  }
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,