Allow for activeClassName to be passed to Link

[walker-tx]

Without destructuring activeClassName, the element will receive the prop and react will throw an error along the lines of React does not recognize the activeClassName prop on a DOM element..

[LekoArts]

Looks good to me 👍
A follow-up PR could re-implement the exact same thing with getProps
https://reach.tech/router/api/Link

As activeClassName only applies to exact routes afaik, with getProps you also get the active style for subroutes (e.g. "blog" for "blog/2018-10-10-my-blogartictle")

[Rahmon]

LGTM

[pieh]

Thanks @wlockiv!

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