cnitool: support capablity args

[feiskyer]

This PR adds capability args to cnitool, which will help to pass capability args for portmapping plugin (containernetworking/plugins#1).

$ export CAP_ARGS='{
	"portMappings": [
		{
			"hostPort":      9090,
			"containerPort": 80,
			"protocol":      "tcp",
			"hostIP":        "127.0.0.1"
		}
	]
}'
$ cnitool add mynet /var/run/netns/myns

cc/ @squeed @dcbw

[dcbw]

LGTM

[feiskyer]

cc @bboreham

[squeed]

seems reasonable, +1