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/webhooks/mandrill/mandrill_webhooks.go

@@ -21,7 +21,7 @@ func (md *MandrillWebhook) Register(router *mux.Router, acc telegraf.Accumulator
 	router.HandleFunc(md.Path, md.returnOK).Methods("HEAD")
 	router.HandleFunc(md.Path, md.eventHandler).Methods("POST")
 
-	log.Printf("Started the webhooks_mandrill on %s\n", md.Path)
+	log.Printf("I! Started the webhooks_mandrill on %s\n", md.Path)
 	md.acc = acc
 }