Want to add README.md how to customize extractors

[miyanokomiya]

The only way to override the extractors for each extensions: ['html', 'vue', 'js'] is a functional syntax.
Because those extensions have been set by default and purgecss uses only one extractor for each extentions.
https://github.com/FullHuman/purgecss/blob/bc9f3b5b9d670832f0d1cca7bc7facfb253fb60b/lib/purgecss.js#L704-L714

This extractor cannot work.

  extractors: [
    { extractor: MyExtracotor1, extensions: ['vue'] }
  ]

We must write configs as below to make our extractors work.

  extractors: () => [
    { extractor: MyExtracotor1, extensions: ['vue'] }
  ]

[manniL]

Go ahead 👍