Add support to get downloaded file when its name is not known.

[TomaszG]

It would be useful to have an option to parse downloaded files when their name is not known at the moment of download. I tried to create a helper function (task) for that which returns the latest file, however, it's not reliable as it's executed when file is not there yet:

// plugins/index.js
const getLastDownloadFilePath = () => {
  const dirPath = 'cypress/downloads';
  const filesOrdered = readdirSync(dirPath)
    .map(entry => path.join(dirPath, entry))
    .filter(entryWithPath => lstatSync(entryWithPath).isFile())
    .map(fileName => ({ fileName, mtime: lstatSync(fileName).mtime }))
    .sort((a, b) => b.mtime.getTime() - a.mtime.getTime());

  return filesOrdered.length ? filesOrdered[0].fileName : false;
};

I believe it may be useful to have the ability to:

1. wait for file download,
2. get the name of the downloaded file.

These two should allow handling such cases.