Add support for impersonating groups with flag

[pratikkumar-mohite]

Adding support for impersonation of groups by passing --as-group through to kubectl exec/get secret cli. We use impersonation as cluster-admins to elevate access to cluster secrets and cluster exec, kubectl supports this with the --as-group flag and supporting this in view-secret makes this much easier to utilize with kubernetes RBAC.

kubectl view-secret --as=pratikkumar.mohite --as-group=system:masters  default-secret                                                         
Error: unknown flag: --as-group

[pratikkumar-mohite]

@elsesiy can you assign this to me, also I've raised the PR #43 could you verify and merge?

[elsesiy]

Implemented via #43