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

Start with the mat-select multiple-selection entry point and review the issue’s linked Bootstrap Select example for the intended interaction. Define how top-level Check All and Uncheck All options should behave, then verify that selecting or clearing all options works in the multiple-selection panel and remains consistent with Angular Material conventions.

Written by the indexing model from the issue text.