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 中没有指定文件或测试。首先定位 MatMenu overlay 的实现，并检查它当前如何控制菜单的大小和位置；对比 AngularJS 菜单的宽度选项。完成的标准应是：为全尺寸移动端菜单定义并测试一种方案，或使 overlay 的高度、宽度和位置可配置。

由索引模型根据 Issue 内容生成。