Add group-focus variant

[adamwathan]

This PR adds support for a new group-focus variant that works just like group-hover, except for focus.

This is useful when you want to add custom focus style to a button or link that has some nested child you want to style in a specific way, for example, changing the color of an icon inside of a button when the button is focused:

<button class="group text-gray-600 focus:bg-gray-100 focus:text-gray-700 ...">
  <svg class="h-6 w-6 text-gray-400 group-focus:text-gray-500 ...">
    <!-- ... -->
  </svg>
  Submit
</button>

This variant is currently not enabled by default for any utilities, but can be enabled in the variants section of your config file.

[sampotts]

Cheers for doing this 👍