function parser(lang, opts) {
  lang = lang || opts.lang;
  if (engines.hasOwnProperty(lang)) {
    return engines[lang];
  }
  return engines.yaml;
}
Determine the correct the parser to use

@param  {String} `lang` Use this if defined and it exists
@param  {Object} `opts` Otherwise, fall back to options.parser or js-yaml
@return {Function}