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/datadog/datadog.go

@@ -92,7 +92,7 @@ func (d *Datadog) Write(metrics []telegraf.Metric) error {
 				metricCounter++
 			}
 		} else {
-			log.Printf("unable to build Metric for %s, skipping\n", m.Name())
+			log.Printf("I! unable to build Metric for %s, skipping\n", m.Name())
 		}
 	}