feat: add -auto-transmit flag to start transmitting on connect

When -auto-transmit is passed, barnard automatically keys up the
microphone as soon as the connection sync completes. Useful for
testing, bots, and unattended operation.

Usage:
  barnard -auto-transmit -server=mumble.example.com -username=test_bot

barnard.go

@@ -28,8 +28,9 @@ type Barnard struct {
 	TLSConfig tls.Config
 
 	Stream    *gumbleopenal.Stream
-	Tx        bool
-	Connected bool
+	Tx           bool
+	AutoTransmit bool // auto-start transmission on connect
+	Connected    bool
 
 	Ui              *uiterm.Ui
 	UiOutput        uiterm.Textview