feat: add ruleOpts to override the default preprocessor rule options

[felipeptcho]

I would like to be able to change the test property of the preprocessor webpack rule or add an exclude property. Adding a ruleOpts parameter that overrides the default rule object solves my problem.

Example:

// doczrc.js
import { css } from 'docz-plugin-css'

export default {
  plugins: [
    // enable CSS Modules when using ".module.scss" file extension
    css({
      preprocessor: 'sass',
      cssmodules: true,
      ruleOpts: {
        test: /\.module\.scss$/,
        exclude: [/node_modules/],
      },
    }),
    // disable CSS Modules when using ".scss" file extension
    css({
      preprocessor: 'sass',
      cssmodules: false,
      ruleOpts: {
        test: /\.scss$/,
        exclude: [/node_modules/, /\.module\.scss$/],
      },
    }),
  ],
}