feat(cli-service): add assetsDir option to specify assets directory

[jkzing]

// vue.config.js
module.exports = {
  outputDir: 'dist',
  assetDir: 'static'
}

output files:

- dist
  - static
    - js
      app.js
    - css
      app.css
    - img
      image.png
  index.html

[yyx990803]

👍