feat(SnackBar): allow to define a container in which to render the snackbar

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

#### What is the expected behavior?
There should be a setting to define a container in which the snackbar is positioned in.

#### What is the current behavior?
SnackBar is always positioned based on the viewport.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-ddtruw

#### What is the use-case or motivation for changing an existing behavior?
I have an app with a static main toolbar at the top. When i position the snackbar to "top right" it will be shown above the main toolbar (see stackblitz example). I would like it to be shown at the top right of the container below the main toolbar (".basic-container" in my stackblitz example).

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
angular 4.4.2
angular-material 2.0.0 - Beta12
Chrome 61.0.3163.100

#### Is there anything else we should know?
MatSnackBarConfig has a viewContainerRef property. At first i thought this would be exactly what i need but when i tried to change it, it didn't seem to do anything. In the documentation of the MatDialog component (which also has a viewContainerRef property) it is stated that this property does not affect the place where it is rendered. I assume the same goes for MatSnackBar.

Beginne mit der verlinkten StackBlitz-Reproduktion und untersuche MatSnackBarConfig.viewContainerRef, wobei du sein Verhalten mit der angeforderten .basic-container-Platzierung vergleichst. Erledigt ist es, wenn eine Einstellung ermöglicht, dass die SnackBar innerhalb des ausgewählten Containers statt im Viewport gerendert und positioniert wird.

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