feature: add get method to get all selected chips

[hamzament]

Instead of providing my own way of handling selected chips state I should be able to get all selected chips from the chipKey I provide to the widget.

Exactly like using a TextEditingController in a TextField to get the text the user inputs.

[nlfiedler]

Thanks for the suggestion, I'm definitely open to pull requests. If the change can be made in a backward compatible fashion, that would be best, but not required.