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/github/github_webhooks.go

@@ -17,7 +17,7 @@ type GithubWebhook struct {
 
 func (gh *GithubWebhook) Register(router *mux.Router, acc telegraf.Accumulator) {
 	router.HandleFunc(gh.Path, gh.eventHandler).Methods("POST")
-	log.Printf("Started the webhooks_github on %s\n", gh.Path)
+	log.Printf("I! Started the webhooks_github on %s\n", gh.Path)
 	gh.acc = acc
 }
 
@@ -58,7 +58,7 @@ func (e *newEventError) Error() string {
 }
 
 func NewEvent(data []byte, name string) (Event, error) {
-	log.Printf("New %v event received", name)
+	log.Printf("D! New %v event received", name)
 	switch name {
 	case "commit_comment":
 		return generateEvent(data, &CommitCommentEvent{})