Html classes

Hi.

I've added an attribute to the MenuItem class to store html attribute one might want to reuse in templates. I'm thinking this could come in handy for styling or specific js targetting or some other uses i can't think of right now.

I thought about adding a convenience __getattr__ method which could map to the attribute dict for terser syntax in the templates. This doesn't seem too important, but if you like the idea i'll gladly add it to the pull request.

I also quickly tweaked the exemple project to show how this could be used with a dead simple use case.