Allow more conventional --namespace / -n flag placement

[mcandre]

Hey, I don't know if this is a problem with kpexec specifically, or more likely the kubectl plugin system. But it's annoying to not be able to use --namespace / -n before the pexec subcommand.

When operating on a lot of different namespaces, it is practical to declare the namespace early rather than late in kubectl commands.

[ssup2]

@mcandre I've known this issue. But this is the limitation of kubectl. This is one of the reasons why I developed kpexec as a standalone binary before kubectl plugin.