docs: add babel-loader example

✅ Closes: #171

examples/babel-loader/.babelrc.js

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+module.exports = {
+  "presets": [
+    "@babel/preset-env",
+    "@babel/preset-typescript"
+  ]
+}

examples/babel-loader/README.md

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+## babel-loader configuration example
+
+It's a basic configuration of the plugin and [babel-loader](https://github.com/babel/babel-loader).
+Very similar to the [ts-loader example](../ts-loader), the main difference in the configuration is that we
+enable **syntactic diagnostics**:
+
+```js
+new ForkTsCheckerWebpackPlugin({
+  typescript: {
+    diagnosticOptions: {
+      semantic: true,
+      syntactic: true,
+    },
+  },
+})
+```
+

examples/babel-loader/package.json

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+{
+  "name": "fork-ts-checker-webpack-plugin-babel-loader-example",
+  "version": "0.0.0",
+  "main": "dist/index.js",
+  "license": "MIT",
+  "scripts": {
+    "dev": "webpack-dev-server",
+    "build": "webpack -p"
+  },
+  "devDependencies": {
+    "@babel/core": "^7.6.0",
+    "@babel/preset-env": "^7.6.0",
+    "@babel/preset-typescript": "^7.6.0",
+    "babel-loader": "^8.0.0",
+    "fork-ts-checker-webpack-plugin": "^5.0.0-alpha.9",
+    "typescript": "^3.8.0",
+    "webpack": "^4.0.0",
+    "webpack-cli": "^3.0.0",
+    "webpack-dev-server": "^3.0.0"
+  }
+}

examples/babel-loader/src/index.ts

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+console.log('Hello world');

examples/babel-loader/tsconfig.json

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+{
+  "compilerOptions": {
+    "target": "ES5",
+    "module": "CommonJS",
+    "lib": ["ES5", "ScriptHost"],
+    "moduleResolution": "Node",
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "skipDefaultLibCheck": true,
+    "strict": true,
+    "baseUrl": "./src",
+    "outDir": "./dist",
+    "forceConsistentCasingInFileNames": true,
+    "importsNotUsedAsValues": "preserve" // this is important for proper files watching
+  },
+  "include": ["./src"],
+  "exclude": ["node_modules"]
+}

examples/babel-loader/webpack.config.js

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+const ForkTsCheckerWebpackPlugin = require('fork-ts-checker-webpack-plugin');
+
+module.exports = {
+  context: __dirname,
+  entry: './src/index.ts',
+  resolve: {
+    extensions: ['.ts', '.tsx', '.js'],
+  },
+  module: {
+    rules: [
+      {
+        test: /\.tsx?$/,
+        loader: 'babel-loader',
+        exclude: /node_modules/,
+      },
+    ],
+  },
+  plugins: [
+    new ForkTsCheckerWebpackPlugin({
+      typescript: {
+        diagnosticOptions: {
+          semantic: true,
+          syntactic: true,
+        },
+      },
+    }),
+  ],
+};