增加对多MPLS情景的支持，支持通过ENV或参数禁用MPLS功能

要提交的变更：
	修改：     cmd/cmd.go
	修改：     printer/basic.go
	修改：     printer/printer.go
	修改：     printer/realtime_printer.go
	修改：     trace/icmp_ipv4.go
	修改：     trace/icmp_ipv6.go
	修改：     trace/trace.go
	修改：     util/util.go

cmd/cmd.go

@@ -55,6 +55,7 @@ func Excute() {
 	classicPrint := parser.Flag("c", "classic", &argparse.Options{Help: "Classic Output trace results like BestTrace"})
 	beginHop := parser.Int("f", "first", &argparse.Options{Default: 1, Help: "Start from the first_ttl hop (instead from 1)"})
 	disableMaptrace := parser.Flag("M", "map", &argparse.Options{Help: "Disable Print Trace Map"})
+	disableMPLS := parser.Flag("e", "disable-mpls", &argparse.Options{Help: "Disable MPLS"})
 	ver := parser.Flag("v", "version", &argparse.Options{Help: "Print version info and exit"})
 	srcAddr := parser.String("s", "source", &argparse.Options{Help: "Use source src_addr for outgoing packets"})
 	srcDev := parser.String("D", "dev", &argparse.Options{Help: "Use the following Network Devices as the source address in outgoing packets"})
@@ -286,6 +287,10 @@ func Excute() {
 		}
 	}
 
+	if *disableMPLS {
+		util.DisableMPLS = "1"
+	}
+
 	res, err := trace.Traceroute(m, conf)
 
 	if err != nil {