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.

Bắt đầu với bản tái hiện StackBlitz được liên kết và kiểm tra MatSnackBarConfig.viewContainerRef, so sánh hành vi của nó với cách đặt .basic-container được yêu cầu. Công việc được coi là hoàn tất khi một thiết lập cho phép SnackBar được render và định vị bên trong container đã chọn thay vì viewport.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.