Feature request: add an option to limit the number of items user can select in multi select

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

#### What is the expected behavior?
In multi select, add an option to limit the number of items user can select in multi select

#### What is the current behavior?
User can select unlimited number of items in multi select

#### What is the use-case or motivation for changing an existing behavior?
It is not uncommon to see questions in surveys like: you may choose up to 3 reasons why you like our product

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 2.0.0-beta.8

首先定位 Angular Material 的 multi-select 组件及其现有测试；阅读选择变更的处理方式，以及组件选项的公开方式。完成的标准是：为 multi-select 实现可配置的选择上限，同时保留正常的选择行为，并添加覆盖该上限的测试。

由索引模型根据 Issue 内容生成。