API Addition + Stack Binding Plugin

G'day there, I've run into a need to bind keys where I don't want to clobber existing bindings. This is in a modular system where my options are to extend or replicate other bindings, and obviously I'd prefer to do the former.

The small public API change is for retrieving the actual callback function, and the plugin is the use case where it was necessary.

Have not minified either obviously, assuming you have your build process and preferred tools.