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/conntrack/conntrack.go

@@ -93,13 +93,14 @@ func (c *Conntrack) Gather(acc telegraf.Accumulator) error {
 
 			contents, err := ioutil.ReadFile(fName)
 			if err != nil {
-				log.Printf("failed to read file '%s': %v", fName, err)
+				log.Printf("E! failed to read file '%s': %v", fName, err)
+				continue
 			}
 
 			v := strings.TrimSpace(string(contents))
 			fields[metricKey], err = strconv.ParseFloat(v, 64)
 			if err != nil {
-				log.Printf("failed to parse metric, expected number but "+
+				log.Printf("E! failed to parse metric, expected number but "+
 					" found '%s': %v", v, err)
 			}
 		}