mat-radio-button and mat-checkbox don't line wrap

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

Proposal: change `` or `` to line-wrap its contents by default or add an option.

#### What is the expected behavior?

I can put a text into a `` or `` that is longer than what fits into the viewport.

#### What is the current behavior?

The content in a or does not line wrap. If you have a lot of text in a it gets cut at the end.

A workaround would be
`{{mylongtext}}`

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-uaeetd?file=app/app.component.html

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

I have a pretty narrow window and need to put checkboxes and radio buttons in there.

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

I think all of them.

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

リンクされた StackBlitz の再現を、具体的には app/app.component.html から始め、狭い viewport で長い mat-radio-button と mat-checkbox のコンテンツが折り返されないことを確認します。コンテンツがデフォルトで、またはドキュメント化されたオプションによって折り返され、コンポーネントの既存の動作が維持されれば完了です。

索引モデルが issue の本文から書いたものです。