Major Logging Overhaul

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

plugins/inputs/nsq_consumer/nsq_consumer.go

@@ -62,7 +62,7 @@ func (n *NSQConsumer) Start(acc telegraf.Accumulator) error {
 	n.consumer.AddConcurrentHandlers(nsq.HandlerFunc(func(message *nsq.Message) error {
 		metrics, err := n.parser.Parse(message.Body)
 		if err != nil {
-			log.Printf("NSQConsumer Parse Error\nmessage:%s\nerror:%s", string(message.Body), err.Error())
+			log.Printf("E! NSQConsumer Parse Error\nmessage:%s\nerror:%s", string(message.Body), err.Error())
 			return nil
 		}
 		for _, metric := range metrics {