Add kube-bench self version

[yoavrotems]

Hi all !
I thought about adding self version support to kube-bench in order to know what version kube-bench you are running.
The problem is there is already --version flag to mention the kubernetes version, WDYT ? should we change the existing flags? (could harm existing uses in kube-bench) or should we add something like kube-bench-version flag? Do you see the need for it?

[mozillazg]

@yoavrotems We already have kube-bench version, maybe it's enough.

$ ./kube-bench version
v0.4.0