New user option 'kubed-default-context-and-namespace'

New user option 'kubed-default-context-and-namespace'

* kubed.el (kubed-default-context-and-namespace): New user
option and function of same name.
(kubed-default-context, kubed-default-namespace): New funcs.
(kubed-local-namespace, kubed-local-context)
(kubed-local-context-and-namespace): Use them.  Adjust
calls to 'kubed-local-namespace'.
* kubed.texi (Context and Namespace): New section.