Add `completed` event for stepper / steps

#### Bug, feature request, or proposal:
Proposal

#### What is the expected behavior?
Optionally allow the consumer define if it wants to auto-advance an step when it completes (by using [completed] step property)

#### What is the current behavior?
Nothing happens as soon the step is marked as completed

#### What is the use-case or motivation for changing an existing behavior?
Use Observable based state such as NgRx stores, and in a more functional way allow set the stepper navigation based on state, now I have to subscribe to store and call navigation via an stepper instance in order to handle steps advance, too imperatively and also results in a awful timing issues because when stepper.next() is called and [completed] property update is on its way nothing happens.

#### Is there anything else we should know?
I'm willing to implement such feature. No breaking changes would be introduced.

Commencez par le point d’entrée du composant stepper/steps de Angular Material et examinez la manière dont sont gérées la propriété existante des étapes terminées et la navigation. Clarifiez le contrat d’auto-avancement optionnel, son interaction avec les mises à jour d’état et la couverture de régression nécessaire ; le travail est considéré comme terminé lorsque le comportement est opt-in et ne casse pas la navigation existante.

Rédigé par le modèle d'indexation à partir du texte de l'issue.