Fix PassAfterNonOption not working with positional args

[hchargois]

This makes PassAfterNonOption work correctly (the same as PassDoubleDash), when used with positional args (as opposed to "unspecified" args being returned by Parse).

I'm not sure this is clear enough, so let me give examples:

type Options1 struct {
    Value bool `short:"v"`
}
type Options2 struct {
    Value bool `short:"v"`
    Positional struct {
        Rest []string `required:"yes"`
    } `positional-args:"yes"`
}

Input line	Using Options1, ParseArgs()...	Using Options2, ParseArgs()...
-v -- -g	parses -v, returns ["-g"]	parses -v and sets Rest to ["-g"], returns []
-v arg -g	parses -v, returns ["args", "-g"]	tries to parse -g and fails ❗

I argue that this bottom-right case is wrong, it should instead not try to parse -g but set Rest to ["arg", "-g"]. This PR fixes this.

[jessevdk]

I think this change makes sense, my only worry is that it changes the behavior of commands, which previously would still apply even if PassAfterNonOption was set. I do think the behavior proposed in this PR is the correct one, I just wonder if this will break anyone's working assumptions.

@zimmski: maybe it's time for:

1. Create v1.0.0 before merging this PR (should fix Please consider SemVer 2.0.0 versioning #206)
2. Merge this PR, create v1.1.0 (this is more of an ABI break in a sense, but semver doesn't cover this afaik)
3. Start writing simple release notes, and mention this change in the one for v1.1.0

[jessevdk]

I prefer if this stayed in this method but was just moved up.

[highbass]

Hey Guys,

What happened with this proposed change? i am running into similar issues and my current workaround is just to check if args length > 0 ...