feat: Compositional parsing

[haf]

Now that more and more projects are starting to use Argu (at least for myself, that is true), it would be neat to be able to chain parsing of parameters together, when parameters are used to configure multiple things. Example: I have a canopy test suite that takes configuration both from environment variables and command line flags; it in turn runs on Expecto which takes configuration from both env and flags, as well. Right now, I have to explicitly tell Argu to ignore unknown inputs and manually compose (imperatively) the two parsers.

It would be nice with an API that would let me parse a union of Argument-type DUs.

[ndalchau]

I've been thinking the same. I have also ended up composing two parsers by ignoring unrecognised inputs.