Add support for multiple WebPush instances

This is a feature requests for a class-based update of this amazing library.

For example:

```js
import { WebPush } from 'web-push';

const webPush = new WebPush();
webpush.setVapidDetails(...);
webpush.sendNotification(...);
```

This will allow for using multiple credentials in a single Node.js server. It also feels like cleaner code in my opinion.