Set default options based on arguments.
function setDefaults(plugin, message, opts) {
  if (typeof plugin === 'object') {
    return defaults(plugin);
  }
  opts = opts || {};
  if (message instanceof Error) {
    opts.error = message;
  } else if (typeof message === 'object') {
    opts = message;
  } else {
    opts.message = message;
  }
  opts.plugin = plugin;
  return defaults(opts);
}