Add 'web-ext lint' support (#120)

Author: pdehaan

package.json

@@ -13,6 +13,7 @@
   "devDependencies": {
     "cross-spawn": "^5.1.0",
     "eslint": "^4.0.0",
+    "npm-run-all": "^4.0.2",
     "pontoon-to-webext": "^1.0.2",
     "prettier": "^1.4.4",
     "rimraf": "^2.6.1",
@@ -25,8 +26,10 @@
     "build": "node scripts/build-locales && web-ext build -s src && unzip -l web-ext-artifacts/*.zip",
     "clean": "rimraf web-ext-artifacts 'src/sidebar/vendor/!(.gitkeep)' && npm run postinstall",
     "format": "prettier 'src/**/!(vendor)/*.{css,js}' --single-quote --write",
-    "lint": "eslint src",
-    "postformat": "npm run lint -- --fix",
+    "lint": "npm-run-all lint:*",
+    "lint:js": "eslint src",
+    "lint:webext": "web-ext lint -s src --ignore-files='sidebar/vendor/*.js' --self-hosted",
+    "postformat": "npm run lint:js -- --fix",
     "postinstall": "scripts/postinstall.sh",
     "prebuild": "npm run clean",
     "start": "web-ext run -s src",