Add 3 state option to multiple select

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

#### What is the expected behavior?
Be able to use `Intermediate` inside `mat-option` like you currently can inside `mat-checkbox`

checkbox with the Intermediate set to true:

multiple select

#### What is the current behavior?
The multiple select can only support 2 states.

#### What is the use-case or motivation for changing an existing behaviour?
I want to provide the use a multi select with options for filtering another check box list. If option one is checked in the multi select all corresponding items from the checkbox list should be checked. If the user then unchecks one of those items the multi select should be updated to the intermediate state.

从 Angular Material 的 mat-select 和 mat-option 入口开始，然后将它们当前的两状态行为与 issue 中提到的 mat-checkbox Intermediate 选项进行比较。当多选选项支持 Intermediate 状态，并且对应选择发生变化时状态会更新，工作即完成。

由索引模型根据 Issue 内容生成。