Complex watcher to develop the CSS and templates of this project. Will check the data files, the templates AND the CSS.
function installDevelopmentWatcher (startingFolder, callback) {
  const buildFolder = config.commands.build.output

  // BrowserSync will watch for changes in the assets CSS.
  // It will also create a server with the build folder and the assets.
  bs.init({
    server: [buildFolder, assets],
    port: config.commands.build.port,
    files: path.join(assets, '**/*.css'),
    ui: false,
    open: false
  })

  // Meanwhile, the watcher will monitor changes in the templates
  // and the data files.
  // Then, when a change occurs, it will regenerate the site through
  // the provide callback.
  const templateFolder = path.join(assets, '**/*.html')
  const dataFolder = path.join(startingFolder, '**/data.xml')

  const watcher = createWatcher(templateFolder, dataFolder)

  watcher.on('change', () => {
    callback()
    bs.reload()
  })
}