docs: function load (routes) {
    if (!Array.isArray(routes) || routes.length === 0) {
      throw new Error('argument should be an array not empty.')
    }
  
    if (!Array.isArray(routes[0])) {
      routes = [routes]
    }
  
    return routes.map(route => _getFiles(route))
                 .reduce((acc, routes) => acc.concat(routes), [])
  }