Answered: Spoiler answer

I don't know if there is a plugin that provides a "spoiler" feature, but with a markdown editor you can (relatively) easily emulate it. Add a style like this to your stylesheet:

.qa-post-content span.spoiler:not(:hover),
.qa-post-content span.spoiler:not(:hover) * {
  color: #aaaaaa;
  background-color: #aaaaaa;
}

and people can write something like this in the edit box:

Lorem ipsum <span class="spoiler">dolor sit amet</span>, consectetur adipiscing elit.

which will then display the <span> as a grey box until the user hovers the mouse over it.