code: function parser(lang, opts) {
    lang = lang || opts.lang;
    if (engines.hasOwnProperty(lang)) {
      return engines[lang];
    }
    return engines.yaml;
  }
docs: 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}