feat(config): add Babel-like config format

docs/POSTCSS.md

@@ -18,7 +18,9 @@ new EmberApp(defaults, {
 });
 ```
 
-## Before/After PostCSS Plugins
+## Processing Order
+
+### `before` / `after` PostCSS Plugins
 
 By default, any PostCSS plugins you specify will be applied after the module transformation. To apply a set of plugins beforehand instead, you can pass a hash with `before` and `after` keys. For instance, if you wanted to use [postcss-nested](https://github.com/postcss/postcss-nested) so that you could define a set of global classes as a single block:
 
@@ -37,7 +39,7 @@ new EmberApp(defaults, {
 });
 ```
 
-## Post-Process Plugins
+### `postprocess` Plugins
 
 You can also provide a set of `postprocess` plugins that will run on the file after it has been concatenated.  This is useful for plugins like `postcss-sprites` that behave better when run against a single file. The `postprocess` array will be passed through to the `plugins` option in [`broccoli-postcss`](https://github.com/jeffjewiss/broccoli-postcss#broccolipostcsstree-options); see that package for details.
 
@@ -53,6 +55,41 @@ new EmberApp(defaults, {
 });
 ```
 
+## Configuring Plugins with Options
+
+Most PostCSS plugins can optionally be configured. Conventionally, you can:
+
+- either pass a plugin as is, to not set any options
+- or call it first to provide options.
+
+```javascript
+new EmberApp(defaults, {
+  cssModules: {
+    plugins: [
+      require('postcss-calc') // without any further config
+      require('postcss-preset-env')({ stage: 4 }) // with extra config
+    ]
+  }
+});
+```
+
+ember-css-modules also supports a Babel-like way to configure plugins:
+
+```javascript
+new EmberApp(defaults, {
+  cssModules: {
+    plugins: [
+      require('postcss-calc'), // without any further config
+      [require('postcss-preset-env'), { stage: 4 }] // with extra config
+    ]
+  }
+});
+```
+
+This special config format is useful, if you are using multiple
+[ember-css-modules plugins](./PLUGINS.md), that would like to inter-operate with
+one another and collaboratively construct a final config.
+
 ## Importing Third Party Files
 
 Out of the box, ember-css-modules doesn't provide a way to to include CSS from outside the app or addon in development. Where possible, including these styles by `app.import`ing them from Bower or using a tool like [ember-cli-node-assets](https://github.com/dfreeman/ember-cli-node-assets) is a good practice, since the build pipeline will have to do less work during development, and your users will benefit from better caching in `vendor.css`.