Add eslint support.

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

package.json

@@ -1,4 +1,5 @@
 {
+  "license": "MIT",
   "dependencies": {
     "@rails/webpacker": "^3.5.3",
     "debug-loader": "^0.0.1",
@@ -12,6 +13,12 @@
     "webpack-cli": "^3.0.8"
   },
   "devDependencies": {
+    "eslint": "^5.3.0",
+    "eslint-loader": "^2.1.0",
+    "eslint-plugin-ignore-erb": "^0.1.1",
     "webpack-dev-server": "^2.11.1"
+  },
+  "scripts": {
+    "lint": "yarn run eslint --plugin eslint-plugin-ignore-erb --ext .js,.js.erb app/javascript/src/javascripts"
   }
 }