[Stepper] linear mode requires control for every step

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

#### What is the expected behavior?
When using linear stepper, I don't want to provide a control.

#### What is the current behavior?
You can't switch to a step without control when linear mode is activated.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-rtrzvu

#### What is the use-case or motivation for changing an existing behavior?
Makes linear usage more smooth without need to add unused code and objects.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 2.0.0-beta 12

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

Start by reproducing the linear-stepper behavior in the linked StackBlitz example, then trace the Angular Material stepper implementation and its linear-mode validation. Done means a linear stepper can move to a step without an associated control, while existing controlled steps retain their expected behavior; verify the change with a focused regression check.

Written by the indexing model from the issue text.