Add ability to convert K8s Obj to PSP

[mstemm]

Add the ability to convert a single K8s Object to a Pod Security Policy,
instead of reading all objects from a live cluster.

• Add spf13/cobra to add subcommands "inspect", which covers the
  existing functionality, and "convert", which converts a single K8s
  Object as a yaml file.

• Move the code that generates a PodSecurityPolicy from lists of
  ContainerSecuritySpec/PodSecuritySpec to a standalone package in
  generator/generator.go. It only has a few minor changes:

  • It has a struct so it's more like an object than a standalone
    function.

  • The provided service account is optional. When not provided
    e.g. nil, "secret" is always added as an allowed volume type.

  • When used by the converter, the namespace and serverGitVersion
    are set to default values "default" and "v1.11", which allows
    enforcement of ReadOnly filesystems.

• Error handling at the top level is done by log.Fatalf instead of
  panic(), to make problems like incorrect arguments a bit more
  graceful.

• Add logging at least for the conversion path, showing the files that
  are read and written at debug level.

[Kaizhe]

@mstemm it looks good to me, could you please also update the README.MD file and provides the example with the new capability as well? Thanks!

[Kaizhe]

Please update the README.MD file as well.

[Kaizhe]

Just a minor question, lgtm