Add support for :visited state variant

[thatguychrisw]

This PR adds the visited state variant to Tailwind core. It can be used as follows:

<a class="visited:text-pink" href="#">Visited Link</a>

It should be placed before the hover and active variants in the configuration or it will override their behavior.

This feature was requested in issue #501, not sure if the implementation is what @adamwathan had in mind. Based this solution on #732

[adamwathan]

Thanks @thatguychrisw!