_findConfigFilePath() {
    if (this._argv.c) {
      return this._argv.c;
    }

    try {
      const filePath = path.resolve('./.esdoc.json');
      fs.readFileSync(filePath);
      return filePath;
    } catch (e) {
      // ignore
    }

    try {
      const filePath = path.resolve('./.esdoc.js');
      fs.readFileSync(filePath);
      return filePath;
    } catch (e) {
      // ignore
    }

    return null;
  }
find ESDoc config file.
@returns {string|null} config file path.
@private