What could a "spoiler tag" extension look like?

Extensions
40 
:::::::::::: SPOILER :::::::::::::
Spoiler Text Here etc....
::::::::::::::::::::::::::::::::::

converts to

<div class="spoiler" > 
   <div class="key header" > spoiler </div>
   ... content here...
</div>

http://talk.commonmark.org/t/content-block-inline-syntax/815