Add aria-current="page" to the active link

[phiter]

What problem does this feature solve?

Users that can see the page can have a nice experience of knowing which link is currently active just by looking at a styled active link with the activeClass.

But visually impaired users, those who rely on screen readers, currently have no way of knowing that the currently selected link is the active link.

To fix this, aria-current="page" adds an extra information to the screen reader that the currently focused link is the same as the current page, so if we add that together with activeClass we'll be aiding both sighted and blind users.

What does the proposed API look like?

Basically, when we add activeClass also pass aria-current="page" to the generated anchor.

[posva]

Duplicate of #2116