md-sidenav: make it automatically responsive

_Feature request_
Version: Angular Material 2.0.0-alpha.7 wax-umpire

I'd like to have an option where the sidenav can be responsive automatically, by having a breakpoint where the sidenav is shown by default when the width is higher than X.

This was already implemented in Angular Material (1) as the `md-is-locked-open`: https://material.angularjs.org/latest/api/directive/mdSidenav

Is there any plan to have this? I could submit a PR if this is wanted.

首先查看 md-sidenav 组件的入口点，以及 issue 中链接的 Angular Material 1 的 md-is-locked-open 行为。定义 responsive 选项和 breakpoint 的语义，然后验证 sidenav 的默认可见性是否会在所要求的宽度下发生变化，并确认该行为已由项目的相关测试覆盖。

由索引模型根据 Issue 内容生成。