link: support React Router's <Link/>

I'm building an app that uses React Router. In order to get cauldron styles on my links, I need to use its `` component. However, this breaks React Router as it expects me to use its `` component.

It'd be nice if we could pass a `component={RouterLink}` prop [like Material UI does](https://material-ui.com/guides/composition/#link) to support this.