checkbox group

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

#### What is the expected behavior?
Similar to the multiple select, it should be possible to have a list of check boxes to select multiple options returned in a single array of choices. A required validator would check for at least one selection.

#### What is the current behavior?
Has to be made via select or custom logic.

#### What are the steps to reproduce?

Providing a Plunker (or similar) is the *best* way to get the team to see your issue.
Plunker template: https://goo.gl/DlHd6U

#### What is the use-case or motivation for changing an existing behavior?
Need it quite often for my projects. Sometimes a select drop down is not what customers wish for a multiple selection. Sometimes it is better to see all options without having to open a menu.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Ng4.1.x, MD beta 6

#### Is there anything else we should know?

首先查看现有的多选和复选框组件，以了解它们当前的 API 和验证行为。定义复选框组的行为，使其返回多个已选选项，并要求至少选择一个选项；当这些交互和验证能够一致地工作时，该 issue 即完成。

由索引模型根据 Issue 内容生成。