add partial namespace matching

[gsstuart]

I've found this enhancement to be very useful in accommodating my own ~laziness~ workflow, so figured I'd submit for inclusion if interested : )

Adds a third option, partial, to the validate_namespaces behavior setting, which causes kubie ns <ns> to perform a check for partial matches when an exact namespace match is not found.

For example, on a cluster with an ingress-system namespace, running kubie ns ingress would switch to ingress-system.

Running kubie ns system would bring up the select-or-list interface showing, i.e., gatekeeper-system, ingress-system and kube-system.

The existing true and false values retain their original behavior, with the default remaining true. This change is backwards-compatible and requires no updates to kubie.yaml, unless one chooses to enable partial.