add support for pod level securityContext

[manherna]

Hi! I have created this PR to check if addition of templating for Pod-level SecurityContext could be accepted.
I find this useful as, depending on who is using your helm Chart, they may need to define a more or less strict securityContext depending on their security policies.

Closes #149

[manherna]

I found this code to be too small to make it into it's own function in securityContext package. but if you see fit, I can move it

[arttor]

Hi, can you please add securityContext to one of the pods in test_data/sample-app.yaml (if it is not already exists), regenerate chart examples with

cat test_data/sample-app.yaml | go run ./cmd/helmify examples/app

cat test_data/k8s-operator-kustomize.output | go run ./cmd/helmify examples/operator

and push updated samples to your branch?

[manherna]

Hi @arttor I added those samples and also changed values naming to podSecurityContext, which I think is more appropriate.

[manherna]

@arttor Thanks for the supports and for this great tool!!