Add -q to list to print only container IDs

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

list.go

@@ -47,6 +47,10 @@ in json format:
 
     # runc list -f json`,
 		},
+		cli.BoolFlag{
+			Name:  "quiet, q",
+			Usage: "display only container IDs",
+		},
 	},
 	Action: func(context *cli.Context) {
 		s, err := getContainers(context)
@@ -54,6 +58,13 @@ in json format:
 			fatal(err)
 		}
 
+		if context.Bool("quiet") {
+			for _, item := range s {
+				fmt.Println(item.ID)
+			}
+			return
+		}
+
 		switch context.String("format") {
 		case "", "table":
 			w := tabwriter.NewWriter(os.Stdout, 12, 1, 3, ' ', 0)