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?

Start at the MatTableDataSource entry point and inspect the existing comments describing filterTermAccessor and its uses. Confirm how filtering currently obtains terms, then add the missing overridable implementation and verify that the accessor can be overridden as requested.

Written by the indexing model from the issue text.