Add true "toggle" (checkbox on/off) capability to toggle handlers

[briandominick]

The original idea for toggle handlers included facet-style toggles for classes independent of others. So, a direct show/hide toggle for one class at a time, checkbox style.

I don't think there's any complex logic in here -- if multiple competing classes are assigned to an element/block, we might want the ability to select an all or one paradigm, whereby a class will show only when all classes are toggled on or when at least one is. We probably want to incorporate that as much as we can to leave the options wide open.

This did not make it into #48, but it should get in by 1.0.