Add all the required partials.
function registerPartials (folder) {
  const partialsFolder = path.join(folder, '_includes')

  const partials = fs.readdirSync(partialsFolder)
    .filter(isValidPartial)
    .reduce((result, partial) => {
      const ext = path.extname(partial)
      const fileFullPath = path.join(partialsFolder, partial)
      const data = fs.readFileSync(fileFullPath, 'utf-8')

      // Store as `"filename without extension": content`.
      result[path.basename(partial, ext)] = data
      return result
    }, {})

  handlebars.registerPartial(partials)
}