kubectl-hns: Add "delete" command

[superbrothers]

This PR adds "delete" command that deletes subnamespaces to the kubectl-hns plugin. The command has --allowCascadingDeletion flag to allow cascading deletion of its subsnamespaces.

Closes #307

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[rjbez17]

/ok-to-test
/lgtm

@adrianludwin this lgtm, LMK if you want to take a look.

[rjbez17]

/assign @adrianludwin

[adrianludwin]

/lgtm
/approve

Thanks for this!

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adrianludwin, superbrothers

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