mat-select - need Check All & Uncheck All options (Select All & Deselect All) for multiple selection

#### Bug, feature request, or proposal:
Request: Need options for **Check All** and **Uncheck All** for multiple selection.

#### What is the expected behavior?
We can have 2 options at the top of the options panel to Check all or Uncheck all options in a single click.
Something like this https://silviomoreto.github.io/bootstrap-select/examples/#selectdeselect-all-options

#### What is the current behavior?
Currently user has to click on each and every option in order to check or uncheck all the options available in the select control.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 5

从 mat-select 的多选入口开始，并查看 issue 中链接的 Bootstrap Select 示例，以了解预期的交互方式。定义顶层 Check All 和 Uncheck All 选项应如何工作，然后验证在多选面板中选择或清除所有选项均可正常工作，并且与 Angular Material 的约定保持一致。

由索引模型根据 Issue 内容生成。