Add configure-options settings (fixes #1438)

[snoyberg]

Addresses #1438

Two points for discussion on this PR:

• Should we also be adding configure-options to Pantry snapshots, like we have support for ghc-options?
• Should we attempt to fold the ghc-options values into this field? I think not, since some commands use ghc-options but don't use configure-options (like ghci)

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

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[qrilka]

Regarding that these options are quite similar to ghc-options it looks to make sense to have them supported in snapshots as well. The other question is that they probably won't get used much.
As for merging ghc-options into this field I agree that probably we should keep them separate and it's user's responsibility to check for any conflicts.
The PR looks good to me but I guess it's better to add support to snapshots as a part of it?

[snoyberg]

@qrilka and I followed up in a private chat about this, and decided that we don't want to require tools that process snapshot files to support arbitrary configure options right now. So we're going to leave this PR as-is.