[Table] Feature Request: Make it easier to provide custom data filters to MatTableDataSource

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

Feature Request

#### What is the expected behavior?

I'd like to be able to provide custom data filters.

#### What is the current behavior?

I can only affect how the free text search is executed

#### What are the steps to reproduce?

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

I have a select box, name "Category" that contains a list of categories. User can select one category from this list. The table should filter to show only items in that category.

I also have free form text search and the user may or may not enter something there. (If the free text is empty, the datasource currently does no filtering.)

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

All

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

Thank you for the awesome framework!

まず、MatTableDataSource の実装と、既存のフリーテキストフィルタリングの動作を確認します。任意のテキスト検索と組み合わせてカテゴリーフィルターを指定する方法を検討し、そのうえで、空のテキストに対する動作を壊さずに、選択したカテゴリーでテーブルをフィルタリングできることを確認します。

索引モデルが issue の本文から書いたものです。