Add listunspent RPC call

Returns a brief json summary of each utxo found by calling
ListUnspentWitness in the wallet. The two arguments are the
minimum and maximum number of conrfirmations (0=include
unconfirmed)

cmd/lncli/main.go

@@ -256,6 +256,7 @@ func main() {
 		newAddressCommand,
 		sendManyCommand,
 		sendCoinsCommand,
+		listUnspentCommand,
 		connectCommand,
 		disconnectCommand,
 		openChannelCommand,