`onclick` property does not work

[ccnokes]

The onclick property of a notification instance doesn't work.

let Notification = require('node-mac-notifier');
let n = new Notification('test');
n.onclick = function() { console.log('onclick'); };
n.addEventListener('click', () => { console.log('event listener');  });

The addEventListener callback gets called but not the onclick.

[CharlieHess]

Should be an easy fix, this is just regular ole JS, but I probably won't get to it for a while so feel free to put up a PR.