Allow passing options for vue-docgen-api

[pocka]

Is your feature request related to a problem? Please describe. We now don't have any way to configure vue-docgen-api. What an inconvenience!

Related to: #83

Describe the solution you'd like Add a loader option to pass to vue-docgen-api.

{
  test: /\.vue$/,
  loader: 'storybook-addon-vue-info/loader',
  options: {
    docgenOptions: {
      // the second argument of docgen.parse
      // https://vue-styleguidist.github.io/Docgen.html#api
      alias: { '@assets': path.resolve(__dirname, 'src/assets') },
      resolve: [path.resolve(__dirname, 'src')],
      // ...
    }
  },
  enforce: 'post'
}

Describe alternatives you've considered none