Made the anchor overridable in NavItem and SubNav

My goal here is to be able to swap out the `React.DOM.a` component with another one. Primarily I'm attempting to use the `Link` from [react-router](https://github.com/rackt/react-router). The goal here being able to tie into abnormal routing/linking logic.
I pushed up that change, the good news is that no tests failed! But I agree that it is a breaking change. The only case I can think of would be around additional classes that someone would like to apply to the li specifically. Should we add an `additionalLiClasses` prop to address that, or is that pre-mature optimization?