Add eslint support.

Run with `yarn run lint` or `bin/webpack`.

config/webpack/loaders/eslint.js

@@ -0,0 +1,10 @@
+module.exports = {
+  enforce: 'pre',
+  test: /\.(js)$/i,
+  exclude: /node_modules|vendor/,
+  loader: 'eslint-loader',
+  options: {
+    cache: true,
+    emitWarning: true,
+  }
+}