code: function merge(input, output) {
    const text = fs.readFileSync(input, 'utf8');
    const loaded = yaml.safeLoad(text.toString(), { schema: yamlfiles.YAML_FILES_SCHEMA });
  
    // write to output
    fs.writeFileSync(output, yaml.safeDump(loaded));
    return output;
  }
docs: parse-merge a given yaml file and save it to a
  given output
  
  @param {string} input - input yaml file path
  @param {string} output - output yaml file path
  @returns {string} the output path