Make it easy to generate assets for #2321 (#2354)

* Make it easy to generate assets for #2321

* ignore assets dir

.eslintignore

@@ -1,5 +1,6 @@
 docs/*
 docker/*
+assets/*
 sitespeed-result/*
 lib/plugins/yslow/scripts/*
 lib/plugins/html/assets/js/*

.gitignore

@@ -4,6 +4,7 @@ node_modules
 *.iml
 .vscode
 sitespeed-result
+assets
 configs
 docs/_site
 yarn.lock

package.json

@@ -44,7 +44,8 @@
     "test": "mocha",
     "check-licenses": "tools/check-licenses.js",
     "travis": "npm run eslint-check && npm run lint && npm run test",
-    "build:css": "node-sass lib/plugins/html/src/sass/main.scss > lib/plugins/html/assets/css/index.css && cleancss -o lib/plugins/html/assets/css/index.min.css lib/plugins/html/assets/css/index.css && rm lib/plugins/html/assets/css/index.css"
+    "build:css": "node-sass lib/plugins/html/src/sass/main.scss > lib/plugins/html/assets/css/index.css && cleancss -o lib/plugins/html/assets/css/index.min.css lib/plugins/html/assets/css/index.css && rm lib/plugins/html/assets/css/index.css",
+    "generate:assets": "mkdir -p assets/$npm_package_version && cp -R lib/plugins/html/assets/ assets/$npm_package_version/"
   },
   "engines": {
     "node": ">=10.13.0"