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/outputs/prometheus_client/prometheus_client.go

@@ -167,7 +167,7 @@ func (p *PrometheusClient) Write(metrics []telegraf.Metric) error {
 				continue
 			}
 			if err != nil {
-				log.Printf("ERROR creating prometheus metric, "+
+				log.Printf("E! Error creating prometheus metric, "+
 					"key: %s, labels: %v,\nerr: %s\n",
 					mname, l, err.Error())
 			}