Support indeterminate state

[turnerhayes]

This component might want to represent an indeterminate state; the UX for that is an open question ("knob" in the middle?).

My particular use case is that I'm trying to add the ability for users to get notified through the Notification API; in order to do that, they need to enable the option through my UI, then respond to the browser's prompt to permit notification access, and only then have the option "enabled". In the meantime, I don't want to show the option as "enabled" or "disabled" (strictly speaking, it's disabled until the user gives permission, but having a control that doesn't change state at all when you interact with it is weird). I'm playing with approaches to that UX, and one thought is to have it in "indeterminate" state until the user has granted or denied permission.