feat(compiler-postcss): add postcss compiler

packages/compiler-postcss/README.md

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+# wepy postcss 编译器
+
+## 安装
+
+```
+npm install wepy-compiler-postcss --save-dev
+```
+
+
+## 配置 `wepy.config.js` ,以使用 `cssnext` 为例
+
+```
+const cssnext = require('cssnext);
+
+module.exports = {
+    "compilers": {
+        postcss: {
+            plugins: [
+                cssnext({
+                    browsers:['iOS 9', 'Android 4.4']
+                })
+            ],
+            map: {
+                inline: true
+            }
+        },
+    }
+};
+```
+
+## 参数说明
+配置参数为[processOptions](http://api.postcss.org/global.html#processOptions)及[plugins](http://api.postcss.org/global.html#Plugin)
+
+
+## 已知Bug
+
+使用`cssnano`插件压缩css时，由于其依赖`macaddress`存在全局变量泄露问题，将导致无法完成编译过程。
+问题相关：[Github](https://github.com/webpack-contrib/css-loader/pull/472)
+
+## Contributor
+
+[fsy0718](mailto:[email protected])

packages/compiler-postcss/index.js

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+const postcss = require('postcss')
+
+exports = module.exports = function (options = {}) {
+  const {plugins = [], ...other} = options
+  return function () {
+    this.register('wepy-compiler-postcss', function (node, file) {
+      return postcss(plugins).process(node.content || '', {
+        from: file,
+        ...other
+      }).then(res => {
+        node.compiled = {
+          code: res.css
+        }
+        return node
+      })
+    })
+  }
+}