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/postgresql_extensible/postgresql_extensible.go

@@ -269,9 +269,7 @@ func (p *Postgresql) accRow(meas_name string, row scanner, acc telegraf.Accumula
 	fields := make(map[string]interface{})
 COLUMN:
 	for col, val := range columnMap {
-		if acc.Debug() {
-			log.Printf("postgresql_extensible: column: %s = %T: %s\n", col, *val, *val)
-		}
+		log.Printf("D! postgresql_extensible: column: %s = %T: %s\n", col, *val, *val)
 		_, ignore := ignoredColumns[col]
 		if ignore || *val == nil {
 			continue