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.
You can also have any number of props since it's `Link` component transfers all non-reserved props to a params object building up url parameters. The real question I have here is what does the `li` really need? See the [source](https://github.com/rackt/react-router/blob/master/modules/components/Link.js) for the `Link` component for details.