init file s3 hash
@param  {String} key filepath
@param  {Array} whitelist list of expressions that match against files that should not be deleted

@return {Boolean} shouldDelete whether the file should be deleted or not
@api private
function fileShouldBeDeleted(key, whitelist) {
  for (var i = 0; i < whitelist.length; i++) {
    var expr = whitelist[i];
    if (expr instanceof RegExp) {
      if (expr.test(key)) {
        return false;
      }
    } else if (typeof expr === 'string') {
      if (expr === key) {
        return false;
      }
    } else {
      throw new Error(
        'whitelist param can only contain regular expressions or strings'
      );
    }
  }
  return true;
}