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

Start with the Angular Material autocomplete and chips components, using the Chips autocomplete example linked in the issue as the expected use case. Review the existing selection and closing behavior and any related tests or API documentation you find. Done means a boolean closeOnSelect option controls whether the list closes after selecting an option, with coverage for both settings.

Written by the indexing model from the issue text.