Rename data variable `state` to avoid collisions

Rename `$spoiler.data('state')` to `$spoiler.data('spoiler-state')` on the off chance that another jQuery plugin is using the variable `state`. For instance, maybe a plugin would allow spoilers to be dragged around the page, and store whether the spoiler has been moved yet in `state`. It’s unlikely, but there’s little harm in being careful.

I chose the name `spoiler-state`, but `shrouded-state` or `hidden-state` might also work.