Draft disclosure (show/hide) design pattern

_From @stevefaulkner on May 25, 2015 13:31_

expand/collapse or disclosure pattern is common on the web (HTML native UI is details/summary), but there is no design pattern. Suggest adding one
a few examples:
http://thepaciellogroup.github.io/disclosure-button/
http://codepen.io/stevef/pen/jiCBE
http://webcloud.se/jQuery-Collapse/

_Copied from original issue: w3c/aria#61_
