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/filestack/filestack_webhooks.go

@@ -19,7 +19,7 @@ type FilestackWebhook struct {
 func (fs *FilestackWebhook) Register(router *mux.Router, acc telegraf.Accumulator) {
 	router.HandleFunc(fs.Path, fs.eventHandler).Methods("POST")
 
-	log.Printf("Started the webhooks_filestack on %s\n", fs.Path)
+	log.Printf("I! Started the webhooks_filestack on %s\n", fs.Path)
 	fs.acc = acc
 }