Pass existing props to Link.getProps

[cfal]

Hi, please take a look and let me knows if this makes sense to land :)

This adds a fifth field existingProps to getProps, which passes down the existing props defined on the Link element. This seems to make Link-based components a bit more reuseable since now we can keep existing props that are passed down (eg. <NavLink class="nav-item"> could simply append active to the existing classes). I updated the example to clarify.

Thanks!