Support per-requirements overrides (--install-option and --global-option flags)

[rgov]

This issue discusses a few use cases where it would be useful to be able to specify additional flags to pass to setup.py, which is what pip's --install-option does.

It was recommended that a bug in this repo be filed to hash out the spec changes, but I did not see one.

@numshub proposed this syntax, to which no one objected. I have a preference for considering a different way to specify multiple options, since this complicates how whitespace is treated.

apackage={version = "*", install-option = "--arg1 --arg2"}

[loot-king]

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