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

Comienza leyendo la implementación de mat-select y mat-option para entender cómo se gestiona actualmente la propiedad readonly selected. Confirma el comportamiento de binding previsto para mat-option, incluidos los casos de mat-select en cascada con multiple=true. El trabajo estará terminado cuando selected pueda establecerse o enlazarse según lo solicitado sin romper el comportamiento de selección existente.

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