add `.shown` utility helper

We are using bootstrap less and mixins to extend bootstrap into our own CSS framework for our site, but there is not currently a mixin utility that reverses `.hidden`'s `visibility` property (which seems to be by design).  These utility mixins are a nice way to apply the basic `display: none; ` properties we've been using to show / hide elements, so I think it would be nice to reuse them, but I understand if that does not coincide with the original intent.