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

Commencez par lire l’implémentation du tableau Angular CDK et ses directives existantes pour les lignes et les cellules afin de comprendre comment les sections du tableau sont gérées. Le travail est terminé lorsque les templates peuvent utiliser facultativement cdk-thead, cdk-tbody et cdk-tfoot autour des lignes concernées, y compris des attributs et des classes sur ces éléments.

Rédigé par le modèle d'indexation à partir du texte de l'issue.