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

首先閱讀 mat-select 和 mat-option 的實作，以了解 readonly selected 屬性目前是如何管理的。確認 mat-option 預期的繫結行為，包括 multiple=true 的串接 mat-select 情況。完成的標準是：可以依要求設定或繫結 selected，且不會破壞現有的選取行為。

由索引模型根據 Issue 內容生成。