Transforms config API improvements

[jorenbroekema]

Before:

{
  platforms: {
    css: {
      transformGroup: 'css',
      transforms: ['name/kebab'],
      basePxFontSize: 14,
      files: [{}]
    }
  }
}

After:

{
  platforms: {
    css: {
      transformGroups: ['css'], // allow multiple
      transforms: ['name/kebab'],
      transformOptions: {
        basePxFontSize: 14,
      },
      files: [{}]
    }
  }
}

Up for discussion of course, just wanted to document the idea and track for v5