Missing implementation of filterTermAccessor in MatTableDataSource.

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

Missing implementation of filterTermAccessor in MatTableDataSource.
#### What is the expected behavior?
There should be the implementation of filterTermAccessor in MatTableDataSource so that it can be overridden.

#### What is the current behavior?
There is no implementation of filterTermAccessor in MatTableDataSource. Only comments are added for the uses of the same. I have searched the entire git repo but did not found any implementation.

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

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

从 MatTableDataSource 入口点开始，检查描述 filterTermAccessor 及其用途的现有注释。确认当前筛选如何获取 term，然后添加缺失的可重写实现，并验证 accessor 可以按要求进行重写。

由索引模型根据 Issue 内容生成。