Make interface properties optional

[hloehrmann]

It would be great if the interface properties would be optional so I can type them without implementing all members.

Currently this leads to a typescript compiler error:

 private types: INotyfNotificationOptions[] = [
    {
      type: 'my-custom-type',
      className: 'my-custom-classname',
    }
  ];

I can make a PR if appreciated.