[Tabs] Allow tabs to show vertical labels

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

#### What is the expected behavior?
It display horizontally in any case and I can`t change this behavior from css or even code. I want display it in this way:
http://joxi.net/8An96qkiPVGe2O

#### What is the use-case or motivation for changing an existing behavior?
Sometimes I need to display sidenav (but designed as tabs) aligned to center or as part of inner block, but in column layout. It would be great add some property for tabs such as direction

#### Is there anything else we should know?
In addition, I used tabs for navigation between routes:
```

{{tabLink.label}}

```

I made this possible in this way:
```
.mat-tab-links {
display: flex;
position: relative;
flex-direction: column; //can`t change this filed by myself
}
```

Beginne mit dem md-tab-nav-bar-Einstiegspunkt und dem in der Issue gezeigten .mat-tab-links-Styling und verfolge anschließend, wie Tab-Links angeordnet werden. Das Ziel ist erreicht, wenn Tabs optional vertikal ausgerichtete Beschriftungen verwenden können und dabei das bestehende horizontale Verhalten erhalten bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.