Add params prop to link component

This change allows a params prop to be passed to the Link component as an alternative to passing a separate prop for each route param.  Although you could achieve the same result using transferPropsTo, doing so will also transfer extraneous props that aren't route params, causing isActive to return false when it should return true.
should probably funnel this through Link.getUnreservedProps also, so you don't blow away reserved props on accident.