Add start script (#7)

Easily test changes in Firefox by running `npm start`.

Author: kfarnung

package.json

@@ -6,6 +6,7 @@
     "build": "webpack && web-ext build -s addon --overwrite-dest",
     "lint": "eslint . && web-ext lint -s addon",
     "lint-fix": "eslint . --fix",
+    "start": "web-ext run -s addon",
     "test": "echo \"Error: no test specified\" && exit 1"
   },
   "license": "MIT",