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?

MatTableDataSource のエントリポイントから開始し、filterTermAccessor とその用途について説明している既存のコメントを調べます。現在のフィルタリングでどのように項目を取得しているかを確認し、不足しているオーバーライド可能な実装を追加して、要求どおり accessor をオーバーライドできることを検証します。

索引モデルが issue の本文から書いたものです。