Support additionalData, process only less-files, fix source-map support

With this update we can do:
...
  .pipe(sourceMaps.init())
  .pipe(less({verbose: true}, {
    modifyVars: {
      buttonTextColor: 'white',
      buttonBgColor: '#26a'
    }
  }))
  .pipe(cssBase64())
  .pipe(sourceMaps.write())
...