Add mixin support to core extension...

Allow createPlugin to support composing the API
through mixins via createMixin and applyMixin(s).
Also allow support for commands. Also add core
mixins: data, event, selection. The usage details
will be in forthcoming documentation.

Refactor and expand createPlugin's API-additions
and createInstance logic, with the latter going
into a separate method.
Minor CoffeeScript fix.
Fixup 0d6180e: pages Grunt task.
Add initial editable plugin port with initial test.