code: function scrapeMetadata (html, rules) {
    rules = rules || RULES
    const keys = Object.keys(rules)
    const $ = cheerio.load(html)
    const promises = keys.map(function (key) {
      return scrapeMetadatum($, '', rules[key])
    })
  
    return Promise.all(promises).then(function (values) {
      return keys.reduce(function (memo, key, i) {
        memo[key] = values[i]
        return memo
      }, {})
    })
  }
docs: Scrape each entry in the metadata result dictionary in parallel.
  
  @param {String} html
  @param {String} url
  @param {Object} rules (optional)
  @return {Promise} metadata