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

Empieza leyendo la implementación de la tabla de Angular CDK y sus directivas existentes para filas y celdas, a fin de entender cómo se gestionan las secciones de la tabla. Se considerará terminado cuando las plantillas puedan usar opcionalmente cdk-thead, cdk-tbody y cdk-tfoot alrededor de las filas relevantes, incluidos los atributos y las clases en esos elementos.

Escrito por el modelo de indexación a partir del texto del issue.