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.

Start by reviewing the md-sidenav component entry point and the Angular Material 1 md-is-locked-open behavior linked in the issue. Define the responsive option and breakpoint semantics, then verify that the sidenav's default visibility changes at the requested width and that the behavior is covered by the project’s relevant tests.

Written by the indexing model from the issue text.