Added test for extra postcss plugins

Author: mvsmal

test/fixtures/react-css-modules/applies extra plugins/actual.js

@@ -0,0 +1,3 @@
+import './bar.scss';
+
+<div styleName="a_modified"></div>;

test/fixtures/react-css-modules/applies extra plugins/bar.scss

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+.a {
+  background-color: #ffffff;
+  
+  &_modified {
+    background-color: #000000;
+  }
+}

test/fixtures/react-css-modules/applies extra plugins/expected.js

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+import './bar.scss';
+
+<div className="bar__a_modified"></div>;

test/fixtures/react-css-modules/applies extra plugins/options.json

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+{
+  "plugins": [
+    [
+      "../../../../src",
+      {
+        "generateScopedName": "[name]__[local]",
+        "filetypes": {
+          ".scss": {
+            "syntax": "postcss-scss",
+            "plugins": ["postcss-nested"]
+          }
+        }
+      }
+    ]
+  ]
+}