requiredOption(flags, description, parseArg, defaultValue) {
    return this._optionEx(
      { mandatory: true },
      flags,
      description,
      parseArg,
      defaultValue,
    );
  }
Add a required option which must have a value after parsing. This usually means
the option must be specified on the command line. (Otherwise the same as .option().)

The `flags` string contains the short and/or long flags, separated by comma, a pipe or space.

@param {string} flags
@param {string} [description]
@param {(Function|*)} [parseArg] - custom option processing function or default value
@param {*} [defaultValue]
@return {Command} `this` command for chaining