Add a way to get GHC arguments based on build targets

[mgsloan]

See the discussion on this PR #1737

Not quite sure what this should look like, creating this stub ticket because I think it's something we should have, but closing the PR.

[luigy]

perhaps as part of the show command #1614 ?

stack show ghc-args TARGET

[mgsloan]

Yup, makes sense!

[afcady]

I did something else with this that isn't apparent on that other page... relevant here:

afcady/stack@4d0e46d...afcady:ghci-no-interactive

Or with less context:

afcady@eccd62b

If you don't do that, it's too slow.

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Might get back to this too eventually, but not soon.