Enable eslint caching in development

POC for #740. Haven't found any problem, build times improved about 1s on my project and machine.

Author: viankakrisna

Diff for: packages/react-scripts/config/webpack.config.dev.js

@@ -118,7 +118,8 @@ module.exports = {
           // Point ESLint to our predefined config.
           options: {
             configFile: path.join(__dirname, '../.eslintrc'),
-            useEslintrc: false
+            useEslintrc: false,
+            cache: true
           },
           // @remove-on-eject-end
           loader: 'eslint-loader'