(cordova) union type for RegistrationOptions

According to the API [documentation](https://github.com/phonegap-build/PushPlugin#register) the `register` function can be called using **string** values for **[badge, sound, alert]** attributes. However uppon [source code inspection](https://github.com/phonegap-build/PushPlugin/blob/master/src/ios/PushPlugin.m#L57-L106), all of those attributes work with either **string** or **boolean** values.

A *union type* can be used for this scenario on the [RegistrationOptions](https://github.com/borisyankov/DefinitelyTyped/blob/master/cordova/plugins/Push.d.ts#L66-L68) so that all three of those attributes be defined as `boolean|string`.