MatMenu: full size menu

#### Bug, feature request, or proposal:
It would be nice to have a full-size menu option for mobile or maybe the ability to adjust the x and y locations and size (height/width) of the overlay.

The overlay is being set by the component and forces the size of the menu and the only way to change this is to extend the mdMenu to allow for specific heights and widths of the menu as well as specific positioning. Or in my case create a component from scratch to do this.

#### What is the use-case or motivation for changing an existing behavior?
My main concern with this request is that it's not in the material spec, however the angularjs version of the menu does have width options.

#### Is there anything else we should know?
If this isn't a good feature for the menu, what would be the best approach to creating a full-size mobile menu?

Issue にはファイルもテストも記載されていません。まず MatMenu overlay の実装を見つけ、現在メニューのサイズと位置をどのように制御しているかを確認してください。AngularJS メニューの幅のオプションと比較します。モバイル向けのフルサイズメニュー、または overlay の高さ、幅、位置を設定可能にするための、明確に定義されテストされたアプローチがあれば完了とします。

索引モデルが issue の本文から書いたものです。