Support returning object, update opacity plugin

Author: adamwathan

src/corePlugins/opacity.js

@@ -1,16 +1,17 @@
 const nameClass = require('tailwindcss/lib/util/nameClass').default
 const { asValue } = require('../pluginUtils')
+const { newFormat } = require('../pluginUtils')
 
 module.exports = function ({ jit: { theme, addUtilities, addVariant, e } }) {
   addUtilities({
-    opacity: (modifier, { theme }) => {
+    opacity: newFormat((modifier, { theme }) => {
       let value = asValue(modifier, theme.opacity)
 
       if (value === undefined) {
         return []
       }
 
-      return [[nameClass('opacity', modifier), { opacity: value }]]
-    },
+      return { [nameClass('opacity', modifier)]: { opacity: value } }
+    }),
   })
 }

src/lib/generateRules.js

@@ -132,7 +132,7 @@ function generateRules(tailwindConfig, candidates, context) {
 
     for (let [sort, plugin] of plugins) {
       if (typeof plugin === 'function') {
-        for (let ruleSet of plugin(modifier, pluginHelpers)) {
+        for (let ruleSet of [].concat(plugin(modifier, pluginHelpers))) {
           if (plugin.format !== 'new') {
             let options = {}
             if (Array.isArray(ruleSet)) {