Feature: Lang aliasing

= Problem

- Vue automatically handles postcss by default, there is many issues
around using custom postcss lang option.
- Using postcss without a `lang` attribute causes SFC to loose syntax
highlghting

= Solution

- Enable option to alias langs to loader settings.
  - specify `postcss` in lang and maintain default loader configurations

docs/en/options.md

@@ -121,6 +121,30 @@ module.exports = {
 
   Note this is automatically set to `false` if the `devtool` option is not present in the main Webpack config.
 
+### langAlias
+
+- type: `Object`
+- default: `{}`
+
+  Alias lang attributes. This can be useful for syntax highlighting support (such as postcss) to alias to alternative loaders or default loaders. 
+
+  ``` js
+  // Webpack 2.x config
+  module: {
+    rules: [
+      {
+        test: /\.vue$/,
+        loader: 'vue-loader',
+        options: {
+          langAlias: {
+            postcss: 'css'
+          }
+        }
+      }
+    ]
+  }
+  ```
+
 ### esModule
 
 - type: `boolean`

lib/loader.js

@@ -53,6 +53,8 @@ module.exports = function (content) {
     esModule: true
   }, this.options.vue, this.vue, query)
 
+  var langAlias = options.langAlias || {}
+
   // disable esModule in inject mode
   // because import/export must be top-level
   if (query.inject) {
@@ -480,14 +482,14 @@ module.exports = function (content) {
 
   function getLangString (type, part) {
     if (type === 'script' || type === 'template' || type === 'styles') {
-      return part.lang || defaultLang[type]
+      return langAlias[part.lang] || part.lang || defaultLang[type]
     } else {
       return type
     }
   }
 
   function getRawLoaderString (type, part, index, scoped) {
-    var lang = part.lang || defaultLang[type]
+    var lang = langAlias[part.lang] || part.lang || defaultLang[type]
 
     var styleCompiler = ''
     if (type === 'styles') {

test/fixtures/lang-alias.vue

@@ -0,0 +1,5 @@
+<style lang="postcss">
+h1
+  color: red
+  font-size: 14px
+</style>

test/test.js

@@ -100,7 +100,7 @@ function mockRender (options, data) {
   }
   function e (text = '') {
     return {
-      text:  text,
+      text: text,
       isComment: true
     }
   }
@@ -496,6 +496,27 @@ describe('vue-loader', function () {
     })
   })
 
+  it('lang alias', done => {
+    test({
+      entry: './test/fixtures/postcss.vue',
+      vue: {
+        postcss: {
+          options: {
+            parser: require('sugarss')
+          }
+        },
+        langAlias: {
+          postcss: 'css'
+        }
+      }
+    }, (window) => {
+      var style = window.document.querySelector('style').textContent
+      style = normalizeNewline(style)
+      expect(style).to.contain('h1 {\n  color: red;\n  font-size: 14px\n}')
+      done()
+    })
+  })
+
   it('transpile ES2015 features in template', done => {
     test({
       entry: './test/fixtures/es2015.vue'