lint gulpfile, add eslint config and ignore file for those that using eslint

.eslintrc.js

@@ -0,0 +1,39 @@
+module.exports = {
+  "root": true,
+  "parserOptions": {
+    "parser": "babel-eslint",
+    "ecmaVersion": 2017,
+    "sourceType": "module"
+  },
+  "env": {
+    "browser": true,
+    "node": true,
+    "es6": true
+  },
+  "globals": {
+    "window": true,
+    "location": true
+  },
+  "extends": [
+    "airbnb-base",
+  ],
+  // custom rules here
+  "rules": {
+    // don"t require .vue extension when importing
+    "import/extensions": ["error", "always", {
+      "js": "never",
+      "vue": "js",
+      "mjs": "never"
+    }],
+    "no-param-reassign": ["error", {
+      "props": true,
+      "ignorePropertyModificationsFor": [
+        "event", // for e.returnvalue
+        "response", // for Express responses
+        "item", // for item usually within each loops
+      ]
+    }],
+    // allow debugger during development
+    "no-debugger": process.env.NODE_ENV === "production" ? 2 : 0
+  }
+}