[Build] use MiniCssExtractPlugin instead of ExtractTextPlugin (#936)

package.json

@@ -78,7 +78,6 @@
     "dependency-tree": "^6.1.0",
     "eslint": "^4.19.1",
     "eslint-plugin-vue": "^4.5.0",
-    "extract-text-webpack-plugin": "^4.0.0-beta.0",
     "fast-vue-md-loader": "^1.0.3",
     "gh-pages": "^1.0.0",
     "html-webpack-plugin": "3.2.0",
@@ -93,6 +92,7 @@
     "karma-spec-reporter": "^0.0.32",
     "karma-webpack": "3.0.0",
     "lint-staged": "^7.0.4",
+    "mini-css-extract-plugin": "^0.4.0",
     "mocha": "^5.1.1",
     "postcss": "^6.0.21",
     "postcss-calc": "^6.0.0",