Add --check-only, for staleness checking.

[quasicomputational]

With this option, hpack will not generate the .cabal file, but will
return with a non-zero status if the .cabal file is missing, stale,
or manually modified.

[quasicomputational]

Ping?

[sol]

@quasicomputational sorry for never getting back at this. This was at a time when I was struggling with RSI badly. That's the reason. However, I think we want something like this. Do you want to pick it up again? If not, I'll look into it.

[sol]

I haven't looked at the code, but from a users perspective what I would want to have is something that gives me a non-negative exit code if .cabal is stale. Maybe have --check or --dry-run print a status, but when combined with --silent (should that be --quiet actually?), have it indicate the status by its exit code.