Add build command for firefox

.gitignore

@@ -1,2 +1,3 @@
 .DS_Store
-node_modules/
+node_modules/
+web-ext-artifacts/

package.json

@@ -4,6 +4,7 @@
   "description": "An extension to quickly add conventional comments",
   "main": "index.js",
   "scripts": {
+    "build:ff": "web-ext build --ignore-files *.gif",
     "test": "echo \"Error: no test specified\" && exit 1",
     "lint": "eslint src/**/*.js && web-ext lint",
     "lint_fix": "eslint --fix src/**/*.js"