Allow filtering by guild

tools/listen.go

@@ -38,6 +38,11 @@ func main() {
 }
 
 func messageCreate(s *discordgo.Session, m *discordgo.MessageCreate) {
+	if guild != "" {
+		if m.GuildID != guild {
+			return
+		}
+	}
 	jsonMsg, err := json.Marshal(m)
 	if err != nil {
 		jsonMsg = append(jsonMsg, '0')