Add support for --kubeconfig flag

[sibucan]

The --kubeconfig flag is useful in multi-cluster situations. This change passes the flag along to the kubectl command to support its usage.

(I renamed my binary show-secret to not clobber the existing installation)

$ kubectl --kubeconfig=admin.conf create secret generic test-secret --from-literal="password=123456"
secret/test-secret created
$ kubectl get secrets --kubeconfig=admin.conf test-secret -ojsonpath='{.data.password}';echo ""
MTIzNDU2
$ kubectl show-secret --kubeconfig=admin.conf test-secret --all
password=123456

[elsesiy]

@sibucan Thanks for the PR. Can you add a test case for this too please?

[sibucan]

The existing tests only cover the ProcessSecret() function, which wasn't modified, and currently there are no tests for flags that are passed directly to kubectl. Do you want me to create a new test function for kubectl flags?

[elsesiy]

Thanks for your submission, I had some other minor changes made and pushed it out!