Provide bash, zsh and powershell completions (a breeze with cobra)

[panchoh]

I just discovered this in cobra:

// GenBashCompletionFile generates bash completion file.
func (c *Command) GenBashCompletionFile(filename string) error {...}

There are equivalent functions for zsh and powershell.

I've tried and generated a bash-completion file, it was easy:

main.go:

...
    kapowCmd.GenBashCompletionFile("/tmp/kapow")
...

Then, I sourced the file from bash, and we have completion, yay!