Enhancement request: use generics with IndividualConfig

Hey guys,

thank you for this awesome work!

Currently, we need a custom toast with custom event handlers... So far i found here an approach: https://medium.com/@chen.reuven/ngx-toastr-with-custom-buttons-a23b90ab5a08 (here the mentioned sample: https://stackblitz.com/edit/ngx-custom-toastr-dynamic-buttons?embed=1&file=app/app.component.ts).

To my question: What do you think about using the IndividualConfig as a generic?

export declare class Toast<IC extends IndividualConfig = IndividualConfig> implements OnDestroy {
...
    options: IC;
...
}

In the stackblitz sample

    this.toastRef = this.toastr
      .show<MyCustomConfig>("Test", null, {
....
        // @ts-ignore <- wouldn't be needed
        buttons: this.toastButtons
      })
      .onAction.subscribe(x => {
        alert(`${x.title} is pressed`);
        console.log(x);
      });

the ts-ignore wouldn't be needed (app.component.ts line 36) and we would be able to add custom properties, too! Also, the origin functionality should work like this.

Cause of the default genneric assignment, the current behavior wouldn't be touched => following is still possible:

    this.toastr.show(...)

scttcper / ngx-toastr

Enhancement request: use generics with IndividualConfig #878