Support critical as loglevel

[zenonhun]

This transport has some hardcoded level, however the underlying producer can handle more:
https://github.com/squeeks/glossy/blob/master/lib/glossy/produce.js

`var SeverityIndex = {
'emerg': 0, // Emergency: system is unusable
'emergency': 0,

'alert': 1,                 // Alert: action must be taken immediately

'crit': 2,                  // Critical: critical conditions
'critical': 2,

'err': 3,                   // Error: error conditions
'error': 3,

'warn': 4,                  // Warning: warning conditions
'warning': 4,

'notice': 5,                // Notice: normal but significant condition

'info': 6  ,                // Informational: informational messages
'information': 6,
'informational': 6,

'debug':  7                 // Debug: debug-level messages

};`

Also winston-syslog supports CustomProducer which may support any loglevel to generate syslog messages.

As a winston logger can have multiple transports it shall be possible to set the available loglevels in then flexibly, to get common severity levels.