Autocomplete "closeOnSelect"

#### Bug, feature request, or proposal:
Proposal to add "closeOnSelect: boolean" property.

#### What is the expected behavior?
- User selects an option
- List does not close

#### What is the current behavior?
- User selects an option
- List is closed automatically

#### What is the use-case or motivation for changing an existing behavior?
https://material.angular.io/components/chips/examples
Here is example "Chips autocomplete"
When user selects many items sometimes auto select closing is not useful.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Last version

#### Is there anything else we should know?
Package select2 provided property "closeOnSelect"
@ng-select provided this option also

从 Angular Material 的 autocomplete 和 chips 组件开始，以 issue 中链接的 Chips autocomplete 示例作为预期用例。检查现有的选择和关闭行为，以及你找到的任何相关测试或 API 文档。当 boolean 选项 closeOnSelect 可以控制列表是否在选择某个选项后关闭，并且覆盖两种设置时，即视为完成。

由索引模型根据 Issue 内容生成。