main: add --private flag to loop in

Adding the --private flag informs loopd that the connected node is
considered private (all channels are marked private) and the loop in
server may have trouble routing the payment. To handle this, loopd will
construct some hophints that will be sent to the loopin server to help
route. This commit merely adds the flag to the cli.

cmd/loop/loopin.go

@@ -37,6 +37,12 @@ var (
 		Usage: "Route hints that can each be individually used " +
 			"to assist in reaching the invoice's destination.",
 	}
+	privateFlag = cli.BoolFlag{
+		Name: "private",
+		Usage: "Generates and passes routehints. Should be used " +
+			"if the connected node is only reachable via private " +
+			"channels",
+	}
 
 	loopInCommand = cli.Command{
 		Name:      "in",
@@ -68,6 +74,7 @@ var (
 			labelFlag,
 			verboseFlag,
 			routeHintsFlag,
+			privateFlag,
 		},
 		Action: loopIn,
 	}