[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?

首先在链接的 StackBlitz 示例中复现线性 stepper 的行为，然后跟踪 Angular Material stepper 的实现及其在 linear-mode 下的验证逻辑。完成的标准是：线性 stepper 可以移动到没有关联 control 的步骤，同时现有的受控步骤仍保持预期行为；使用有针对性的回归检查验证此次更改。

由索引模型根据 Issue 内容生成。