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/powerdns/powerdns.go

@@ -110,7 +110,7 @@ func parseResponse(metrics string) map[string]interface{} {
 
 		i, err := strconv.ParseInt(m[1], 10, 64)
 		if err != nil {
-			log.Printf("powerdns: Error parsing integer for metric [%s]: %s",
+			log.Printf("E! powerdns: Error parsing integer for metric [%s]: %s",
 				metric, err)
 			continue
 		}