Add optional replace flag

[lewismarshall]

Tested from clean (uses create):

~/go/src/github.com/UKHomeOffice/kd add_replace[+]> ./bin/kd -n default --replace -f ./test/deployment.yaml 
[INFO] 2018/07/31 11:41:00 main.go:462: deploying deployment/nginx-deployment
[INFO] 2018/07/31 11:41:01 main.go:469: deployment "nginx-deployment" created
[INFO] 2018/07/31 11:41:06 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:07 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:08 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:09 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:10 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:11 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:12 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:13 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:14 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:15 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:16 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:17 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:18 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 3 objects.
[INFO] 2018/07/31 11:41:19 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 2 objects.
[INFO] 2018/07/31 11:41:20 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 1 objects.
[INFO] 2018/07/31 11:41:21 main.go:589: Deployment "nginx-deployment" update in progress. Waiting for 1 objects.
[INFO] 2018/07/31 11:41:22 main.go:586: Deployment "nginx-deployment" is complete. Available objects: 3

With existing object (uses replace):

~/go/src/github.com/UKHomeOffice/kd add_replace[+]> ./bin/kd -n default --replace -f ./test/deployment.yaml 
[INFO] 2018/07/31 11:41:45 main.go:462: deploying deployment/nginx-deployment
[INFO] 2018/07/31 11:41:45 main.go:469: deployment "nginx-deployment" replaced
[INFO] 2018/07/31 11:41:50 main.go:586: Deployment "nginx-deployment" is complete. Available objects: 3
~/go/src/github.com/UKHomeOffice/kd add_replace[+]> kubectl -n default get po

[KashifSaadat]

Changes look good, thanks for the addition!

As @Joseph-Irving suggested, the readme should be extended with some use-cases for this flag as to when it may be needed.

[KashifSaadat]

LG, thanks!