Use plugin options to enable/disable modules

Something like this would allow for more complex configurations in case we need it in the future 👍 

```js
// tailwind.config.js
{
  plugins: [
    require('tailwindcss-gutenberg')({
      colors: true,
      fontSizes: true,
      foregroundColors: true,
      alignments: true,
      admin: true,
    }),
  ],
}
```

Let me know if you'd like me to open a PR with the changes.