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/sysstat/sysstat.go

@@ -203,7 +203,7 @@ func (s *Sysstat) collect() error {
 	out, err := internal.CombinedOutputTimeout(cmd, time.Second*time.Duration(collectInterval+parseInterval))
 	if err != nil {
 		if err := os.Remove(s.tmpFile); err != nil {
-			log.Printf("failed to remove tmp file after %s command: %s", strings.Join(cmd.Args, " "), err)
+			log.Printf("E! failed to remove tmp file after %s command: %s", strings.Join(cmd.Args, " "), err)
 		}
 		return fmt.Errorf("failed to run command %s: %s - %s", strings.Join(cmd.Args, " "), err, string(out))
 	}