static _setDefaultConfig(config) {
    if (!config.includes) config.includes = ['\\.js$'];

    if (!config.excludes) config.excludes = ['\\.config\\.js$', '\\.test\\.js$'];

    if (!config.index) config.index = './README.md';

    if (!config.package) config.package = './package.json';

    if (!('outputAST' in config)) config.outputAST = true;
  }
set default config to specified config.
@param {ESDocConfig} config - specified config.
@private