[Sort] Adjust sort interface to handle swapping out MatSort

#### Bug, feature request, or proposal:
It would be nice to be able to sort on multiple columns.

#### What is the expected behavior?
The user can sort the grid with multiple columns at a time

#### What is the current behavior?
The user can sort the grid with a single column

#### What is the use-case or motivation for changing an existing behavior?
The app I'm working on has a requirement for multi-column sort. I've used other, non-angular, grids in the past and they have supported this.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 4.3.6
Material 2.0.0-beta.10
Typescript 2.5.2

首先阅读标题中引用的排序接口和 MatSort 行为，然后追踪当前的单列排序是如何向 grid 暴露的。完成的标准是 grid 可以同时按多个列排序，同时保持清晰且有文档说明的接口和行为。

由索引模型根据 Issue 内容生成。