Add utility variants in the Utiliity API

@mdo, it would be very helpful to have the possibility to choose whether we need variants for different utility classes inside the API. For example, we could add a transform or shadow property on hover, focus, and so on.

$utilities: (
   "opacity": (
      property: shadow,
      responsive: true,
      **variants: hover, focus**
      values: $shadow-values
   )
);

I think this is one feature that would make this cool API almost complete.

[ydmitry]

Yes, states is better

[mdo]

Added in Beta 1!