refactor: allow overriding email comb selector safelist

[cossssmin]

This PR changes the way removeUnusedCSS behaves, allowing you to completely override its selectors safelist from your config.js. This was necessary so that email client targeting selectors do not show up in all of your project's templates when you just want to use a one-off. They would normally show up because Tailwind CSS is compiled once for all templates, and then is purged on a template-basis.

Maizzle won't remove these selectors in the first pass, but with this change you can now run a quick second pass to remove unnecessary client targeting selectors from templates that don't need them:

// config.production.js

const {removeUnusedCSS} = require('@maizzle/framework')

module.exports = {
  // ...
  events: {
	// Run removeUnusedCSS again, this time overriding the `whitelist` option so that no email client targeting selector is preserved
    afterTransformers(html) {
      return removeUnusedCSS(html, {
        whitelist: [],
      })
    }
  }
}