The default ShellCompDirective can be customized for a command and its subcommands

[albers]

Closes #2209, #2221

#2221 proposed a solution where the custom DefaultShellCompDirective only applied to a given command.

In #2221 (comment), alternative approaches were discussed.
We agreed on a recursive solution, where a change applies to a command and its subcommands.

To ease review, I open a new PR for this approach.

[ccoVeille]

I'm a bit annoyed by something here

We have an exported field with a pointer.

I remember the discussion that occurred in previous PR about that.

But here, the value the user may expect to be able to pass this field when creating an instance of the struct.

But there is a problem, the existing values they may want to pass are constants (with a iota btw)

But then... You cannot pass a reference to a constant.

I mean this cannot be written

_ = cobra.Command{
	DefaultShellCompDirective: &cobra.ShellCompDirectiveNoFileComp
}

So something else need to be found. i think

Either removing the pointer (and use something else like another constant equal to -1), using variables and not constants, adding a setter...