Supporting auto-completion about go-flags

[jessevdk]

Yeah, it would be great to support something like this. For this to be really useful, there are still some things missing.

1. A way to indicate that a flag argument is a file, could be a type Filename string alias
2. A way to complete positional arguments in a certain way (for example as files).

[elazarl]

1. In my way, a simple predefined Filename with FlagComplete function will do.
2. What we could do (if we neglect issue #33 , which allows one to add completion in exactly the same way). Is to add FlagCompletePositional() method to the flag's struct itself.

type opt struct {
    verbose `short:"v"`
}
func (o *opt) FlagCompletePoistional(args []string) []string {
    if len(args) == 0 {
        return []string{"first1", "first2"}
    }
    return []string{"rest1", "rest2"}
}

[jessevdk]

I would feel more inclined to then also fix #33. Additionally, the FlagComplete should accept a string and complete towards partial matches of that string, instead of just all matches. I would also only match flags (--verbose) if there is an initial -, which I think is more in line with how bash completion is normally implemented.

[jessevdk]

I've finally gotten around to implementing this. Please see the docs for information on how it works and how you can enable bash-completion for your application.

[rlmcpherson]

Awesome, thanks for this. Looking forward to using it.