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,
                     Browser::UserTask* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    if (!dict.Get("program", &(out->program)) ||
        !dict.Get("title", &(out->title)))
      return false;
    if (dict.Get("iconPath", &(out->icon_path)) &&
        !dict.Get("iconIndex", &(out->icon_index)))
      return false;
    dict.Get("arguments", &(out->arguments));
    dict.Get("description", &(out->description));
    return true;
  }
};
using atom::JumpListCategory;
using atom::JumpListItem;
using atom::JumpListResult;
template <>
struct Converter<JumpListItem::Type> {
  static bool FromV8(v8::Isolate* isolate,