Add option to track toggled state via body class

[jeremyfelt]

1. We're often able to use :has() like body:has(.toggle-block[aria-expanded="true"]), but have run into a couple cases where older versions of iOS still in use do not support :has()
2. Also, if more than one toggle block exists on a page, it would be nice to have a page-level awareness of which one is toggled.

So, we can add another field that accepts a class name to be assigned to body when that particular toggle is toggled.