code: function deleteDirectoryFiles(directory, filter = () => true) {
    return readDirectoryFiles(directory)
      .then((files) => {
        const quietDelete = f => deleteFile(path.join(directory, f)).catch(() => null);
        const deletingFiles = files.filter(filter).map(quietDelete);
        return Promise.all(deletingFiles).then(deleted => deleted.filter(d => d !== null));
      });
  }
docs: Delete all the files in a directory, applying an optional filter
  @param {string} directory path of the directory to clean
  @returns {Promise<array>} resolve with all the files deleted succesfully