Add JavaScript linting using ESLint and WordPress standards

Commands:

- `npm run lint:js` to display JS coding violations
- `npm run fix:js` to fix JS violations that can be auto-corrected.

.eslintignore

@@ -0,0 +1,4 @@
+node_modules
+vendor
+*.min.js
+js/responsive-menus.js

.eslintrc.js

@@ -0,0 +1,3 @@
+module.exports = {
+	"extends": "wordpress",
+};

package.json

@@ -10,6 +10,8 @@
 		"adm-zip": "^0.4.13",
 		"autoprefixer": "^9.4.2",
 		"chalk": "^2.4.1",
+		"eslint": "^5.15.2",
+		"eslint-config-wordpress": "^2.0.0",
 		"node-wp-i18n": "^1.2.1",
 		"npm-run-all": "^4.1.5",
 		"postcss-cli": "^6.0.1",
@@ -20,6 +22,8 @@
 		"autoprefixer": "postcss style.css lib/**/*.css --use autoprefixer --replace --no-map --verbose",
 		"makepot": "node .scripts/makepot.js",
 		"lint:css": "stylelint \"style.css\" \"lib/gutenberg/*.css\" \"lib/woocommerce/*.css\"",
+		"lint:js": "eslint .",
+		"fix:js": "eslint . --fix",
 		"build:export-zip": "git archive -o genesis-sample.zip --prefix=genesis-sample/ HEAD",
 		"build:md-to-txt": "node .scripts/md-to-txt.js",
 		"zip": "run-s build:export-zip build:md-to-txt"