Add possibility to check if modalRef is shown

Bug description:

Pure bootstrap has event listener to check when modal is shown. NgbModal – no 🙁
But I think, this is a good idea (for example in some bad UI cases i need to hide other modals, and i need to check whether current modal is shown or hidden)

Version of Angular, ng-bootstrap, and Bootstrap:

Angular: 4.0.1

ng-bootstrap: 1.0.0-alpha.26

Bootstrap:

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