OpenType feature menu concept

OpenType features are really powerful, and often critical to quality typesetting — yet I have *never* seen a good interface for using them. So I'm working on one, to be employed in Typekit Practice lessons as a proof of concept.

I expect this to work like the "Change font" menu, revealed by an icon and scoped to individual examples: http://practice.typekit.com/lesson/selecting-typefaces-for-body-text/