Provide extensions points for menu items and toolbar buttons

[oliverw]

I would be nice if MochaUI would provide extension points for menu items and toolbar buttons that would allow to map custom attributes from menu item/toolbar button option to the injected DOM element.

My concrete use case for this is that I'm using knockout.js's awesome databinding and thus need to carry over my custom "databind" attributes on MochaUI toolbar buttons and menu items to a "data-bind" attribute on the generated DOM elements.

I am not going to work on any new features for Mocha right now. I am going for a stable version now. If you want to submit something I will be willing to see if I can patch it in.