There seems to be an inconsistent typing of the shutdown function.

The definition file specifies the shutdown function as follows

export function shutdown(cb?: (error: Error) => void): void | null;

however the function seems to require a cb function (See https://github.com/aaron-edwards/log4js-node/blob/master/lib/log4js.js#L106 & https://github.com/aaron-edwards/log4js-node/blob/master/lib/log4js.js#L112)