Add basic set of opacity utilities

src/generators/opacity.js

@@ -0,0 +1,10 @@
+import _ from 'lodash'
+import defineClass from '../util/defineClass'
+
+export default function ({ opacity }) {
+  return _.map(opacity, (value, modifier) => {
+    return defineClass(`opacity-${modifier}`, {
+      'opacity': value,
+    })
+  })
+}