Add biOption for two-argument options

[roberth]

This is a potential solution to #271 and #284, observing that both issues only seem to want to parse a fixed number of two arguments per option. That certainly applies to my use case as well.

I don't think this solution is pretty, but it is backwards compatible and it does the job.

[HuwCampbell]

Interesting, but yeah I agree it's a bit janky.

So I'm not sure (using GADTs is almost certainly a no though as I'm trying to keep optparse very lean).

I think if people like yourself are really needing this, the best course of action is to resurrect #284, but make the behaviour opt-in under a new Mod OptionFields a modifier which turns on the variable length consumption.

That way people can't easily foot-gun themselves, and no-one's code will break.

[MagicRB]

@roberth did you not end up needing this, trying to translate a bash script into Haskell and need to parse --override-input nixpkgs something, you'll never guess what package manager this is for.

[roberth]

@MagicRB 😆

I ended up maintaining a fork with a small patch, as the proposed alternative turned out to be a major refactor that's too big for a mere mortal with a job to do.
The fork was supposed to be a temporary thing before I circled back to this, but you know how things go.

[MagicRB]

The most temporary solutions become the most permenant, or something. I'll use your fork then, because --override-input nixpkgs=foobar is everything but nice