CDK Table: `cdk-thead` directive

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

#### What is the expected behavior?
CDK Tables - optional exposure of the ``/``/`` in the template via a `cdk-thead`/`cdk-tbody`/`cdk-tfoot` directive.

I would expect the something like the following template to be supported:

``` html

User name
{{row.username}}

```
[Expanded on from the template provided in the overview]

#### What is the current behavior?
The tags are not [documented as?] something that you can place inside the cdk-table template.

#### What are the steps to reproduce?
n/a

#### What is the use-case or motivation for changing an existing behavior?
CDK Tables are awesome for un-opinionated tables, especially given their easy integration with external CSS frameworks like bootstrap. Lack of access easy access to ``/``/`` in the template feels then like something of a gap given what CDK-tables seem to be trying to achieve, and its ability to support a CSS framework that applies classes to these tags.

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

#### Is there anything else we should know?
n/a

Beginne damit, die Implementierung der Angular-CDK-Tabelle und ihre vorhandenen Zeilen- und Zell-Direktiven zu lesen, um zu verstehen, wie Tabellenabschnitte behandelt werden. Erledigt ist die Aufgabe, wenn Templates optional cdk-thead, cdk-tbody und cdk-tfoot um die relevanten Zeilen herum verwenden können, einschließlich Attributen und Klassen auf diesen Elementen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.