[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!

Beginne damit, die Implementierung von MatTableDataSource und ihr bestehendes Freitext-Filterverhalten zu finden. Ermittle, wie ein Kategorie-Filter zusammen mit einer optionalen Textsuche bereitgestellt werden könnte, und überprüfe anschließend, dass die Tabelle nach der ausgewählten Kategorie filtern kann, ohne das Verhalten bei leerem Text zu beeinträchtigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.