function splitByAuthor(files) {
  let result = {};
  files.forEach(function(file) {
    let authors = file.authors || [];
    authors.forEach(function(author) {
      if (!result[author]) {
        result[author] = [];
      }
      result[author].push(file);
    });
  });
  return result;
}
Splits a list of files by author

@param {Array} files The array of files split.
@return {Object} A list of files split by author.