Support for basic variant generator plugins

Allows you to write a plugin that registers a new variant but only
allows you to modify the selector (like what our built-in generators
do.)

Next steps are to support variants that wrap rules with at-rules
(like @supports for example), variants that can modify properties
(as opposed to just selectors), and to give variant plugin authors
control over how responsive variants interact with their own variants.

src/util/processPlugins.js

@@ -14,6 +14,21 @@ function parseStyles(styles) {
   return _.flatMap(styles, style => (style instanceof Node ? style : parseObjectStyles(style)))
 }
 
+function generateVariantFunction(generator) {
+  return (container, config) => {
+    const cloned = container.clone()
+
+    cloned.walkRules(rule => {
+      rule.selector = generator({
+        className: rule.selector.slice(1),
+        separator: escapeClassName(config.options.separator),
+      })
+    })
+
+    container.before(cloned.nodes)
+  }
+}
+
 export default function(config) {
   const pluginComponents = []
   const pluginUtilities = []
@@ -60,6 +75,9 @@ export default function(config) {
 
         pluginComponents.push(...styles.nodes)
       },
+      addVariant: (name, generator) => {
+        pluginVariantGenerators[name] = generateVariantFunction(generator)
+      },
     })
   })