feat/namespace-view-or-switch

[mosheavni]

If you type: :Kubens without args, open the namespace view
If you type: :Kubens some-ns, change to it

remove the duplicated function to change NS since we already have it in the namespace view

future thoughts:

• move the list_namespaces to the namespace view
• do the same for :Kubectx

[mosheavni]

with this: d764949
I improve the :Kubens completion by saving namespaces in the namespace view.

[Ramilito]

Nice!