Feature request: Add shell completions

[johanneskastl]

Dear maintainers,

very interesting project you got there. Will definitely play around with it.

If it is not too much trouble, could you add a command that generates completion files for shells like bash, zsh, fish and powershell?

Most kubernetes-related tools seem to have them, I was told it is easy to add them (but I am no go programmer). And they make life a little easier for the users.

Thanks in advance.

Kind Regards,
Johannes