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/graphite/graphite.go

@@ -85,7 +85,7 @@ func (g *Graphite) Write(metrics []telegraf.Metric) error {
 	for _, metric := range metrics {
 		gMetrics, err := s.Serialize(metric)
 		if err != nil {
-			log.Printf("Error serializing some metrics to graphite: %s", err.Error())
+			log.Printf("E! Error serializing some metrics to graphite: %s", err.Error())
 		}
 		bp = append(bp, gMetrics...)
 	}
@@ -102,7 +102,7 @@ func (g *Graphite) Write(metrics []telegraf.Metric) error {
 		}
 		if _, e := g.conns[n].Write([]byte(graphitePoints)); e != nil {
 			// Error
-			log.Println("ERROR: " + e.Error())
+			log.Println("E! Graphite Error: " + e.Error())
 			// Let's try the next one
 		} else {
 			// Success