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.
Note, y'all state on the project readme:

> Under active development - APIs will change.