code: function readStylesheets(files, outputBanner) {
      return Promise.all(files.map((filename) => {
          if (isURL(filename)) {
              return new Promise((resolve, reject) => {
                  request({
                      url: filename,
                      headers: { 'User-Agent': 'UnCSS' }
                  }, (err, response, body) => {
                      if (err) {
                          return reject(err);
                      }
                      return resolve(body);
                  });
              });
          } else if (fs.existsSync(filename)) {
              return new Promise((resolve, reject) => {
                  fs.readFile(filename, 'utf-8', (err, contents) => {
                      if (err) {
                          return reject(err);
                      }
                      return resolve(contents);
                  });
              });
          }
          throw new Error(`UnCSS: could not open ${path.join(process.cwd(), filename)}`);
      })).then((res) => {
          // res is an array of the content of each file in files (in the same order)
          if (outputBanner) {
              for (let i = 0, len = files.length; i < len; i++) {
                  // We append a small banner to keep track of which file we are currently processing
                  // super helpful for debugging
                  const banner = `/*** uncss> filename: ${files[i].replace(/\\/g, '/')} ***/\n`;
                  res[i] = banner + res[i];
              }
          }
          return res;
      });
  }
docs: Given an array of filenames, return an array of the files' contents,
  only if the filename matches a regex
  @param  {Array}   files  An array of the filenames to read
  @return {Promise}