add eslint rule

Makefile

@@ -4,10 +4,18 @@ PORT = 3000
 
 help:
 	@echo make lang: build translation files
+	@echo make eslint: use ESLint to check code standard conformence
 	@echo make start-dev-server: start a development server
 
 lang:
 	cd l10n; make
 
+eslint:
+	-npx eslint server/trivabble-server.js
+	-npx eslint public/alert.js
+	-npx eslint public/config.js
+	-npx eslint public/l10n.js
+	-npx eslint public/trivabble.js
+
 start-dev-server:
 	cd server && make start-dev-server