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

Comece lendo a implementação de mat-select e mat-option para entender como a propriedade readonly selected é gerenciada atualmente. Confirme o comportamento de binding pretendido para mat-option, incluindo casos de mat-select em cascata com multiple=true. O trabalho estará concluído quando selected puder ser definido ou associado conforme solicitado sem quebrar o comportamento de seleção existente.

Escrita pelo modelo de indexação a partir do texto da issue.