Feature request: pull to refresh

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

#### What is the expected behavior?
When the user is at the top of a list and pulls, a refresh icon slides in. When the icon is fully visible and the user releases, the list is refreshed.
Material spec can be found here: https://material.io/guidelines/patterns/swipe-to-refresh.html

#### What is the use-case or motivation for changing an existing behavior?
We are building a web application using Angular Material, which we will be shipping as an app. We wish to conform to the Material spec and thus allow our users to refresh a list by pulling down.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular: 5.2.3
Material: 5.1.1
TypeScript: 2.7.1

该 issue 未指定任何仓库文件、测试或入口点。首先查看链接的 Material swipe-to-refresh 规范以及现有的 Angular Material 组件模式；在实现之前定义组件和 API 的范围。完成标准是：从顶部向下拉动列表，并在刷新图标完全可见后松开，列表随即刷新。

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