nbToasterService: Ability to add custom close button & On Close event

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x ] feature request

### Issue description

**Current behavior:**
nbToasters can only currently be closed by
a) clicking anywhere on the toaster
b) setting a timeout

In addition to this we can't trigger a custom event after the toaster closes.

In this particular instance we are building a PWA and showing using the toast to tell the user a new version is available. We want the reload event to only take place after the user clicks 'ok' on the toast.

**Expected behavior:**
angular2-toaster allows the following additional configs to allow these features

showCloseButton: boolean | {
[key: string]: boolean;
};
closeHtml: string;
along with a callback called onHideCallback() to allow a custom event on toaster close.

Would be great if you could build these features into nbToaster