fill prefix with date, if enabled and empty.

Date is in milliseconds or UTC string format. 
The behaviour can be modified with two boolean style fields 'fillDateIfEmpty', 'useMS'.

e.g. 
var log = require('npmlog')
log.prefixStyle = { fillDateIfEmpty: true, useMS: true, fg: 'green' }
log.info('', 'show me when it happened');