Extracted LinkMixin from Link

The goal here is to be able to allow for a [react-bootstrap](https://github.com/react-bootstrap/react-bootstrap) `NavItem` to utilize the power of `Link`. Virtually removing all the duplicated code in [react-router-bootstrap](https://github.com/mtscout6/react-router-bootstrap) in particular for the [`NavItemLink` component](https://github.com/mtscout6/react-router-bootstrap/blob/master/src/NavItemLink.cjsx).