mat-select - Allow mat-option selected property to be settable

#### Bug, feature request, or proposal:
Feature request

#### What is the expected behavior?
To be able to set the mat-option selected property

#### What is the current behavior?
Currently the mat-option selected property is readonly and you have to programatically select options.

#### What is the use-case or motivation for changing an existing behavior?
Being able to bind to the selected property would be a cleaner implementation for a user versus having to do it manually, especially in cases where using cascading mat-select with multiple=true (harder to manage)

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 4.4.3
Material 2.0.0-beta.11
Windows 10
TypeScript - 2.5.2
All browsers

首先阅读 mat-select 和 mat-option 的实现，以了解 readonly selected 属性当前是如何管理的。确认 mat-option 预期的绑定行为，包括 multiple=true 的级联 mat-select 情况。完成的标准是：可以按要求设置或绑定 selected，且不会破坏现有的选择行为。

由索引模型根据 Issue 内容生成。