Navigation drawer - Mini variant behavior

#### Feature request:
Add the "Mini variant" behavior of the Navigation drawer.
As defined in the Google Material guidelines:
https://material.google.com/patterns/navigation-drawer.html#navigation-drawer-behavior


#### What is the expected behavior?
When toggled, the navigation drawer change its width.
It's not fully hidden.

#### What is the current behavior?
Only two states are available: displayed, hidden

#### What are the steps to reproduce?
Create a sidenav and toggle it on and off.
The sidenav can only be opened or closed.
No intermediate state.

#### What is the use-case or motivation for changing an existing behavior?
Desktop users need to quickly change the current view.
The sliding effect of the sidenav could be annoying when used a lot.
It's common for desktop apps to keep the navigation opened by default.
But in some cases the icons are self explanatory so no need to display the associated labels.

#### Which versions of Angular, Material, OS, browsers are affected?
Angular2

#### Is there anything else we should know?
A similar feature request was submitted for Angular 1:
https://github.com/angular/material/issues/3158

まず、既存の Angular Material sidenav 実装と、現在の開閉切り替えの経路を確認します。その動作を、リンクされている Google Material mini-variant ガイドラインと比較します。完了条件は、既存の表示状態と非表示状態を維持しながら、drawer が独立した縮小幅の状態をサポートすることです。

索引モデルが issue の本文から書いたものです。