Editable default command

[Hayao0819]

In #1458 , brianstrauch said

A good long term solution would be to allow users to edit these descriptions to match their preferred style.

I know SetHelpCommand(ref) but I would like to only change Short of help command.

My simplest idea is change Command.helpCommand to Command.HelpCommand or add function such the following

func (c *Command) GetHelpCommand(cmd *Command) {
    return &c.helpCommand
}