stack_install: Override Cabal flags

[aherrmann]

Is your feature request related to a problem? Please describe.
Hazel allows users to override per package Cabal flags. The new stack_install does not provide this feature. An example use-case would be the use of integer-simple instead of integer-gmp which requires passing additional Cabal flags to various Stackage packages. See here.

Describe the solution you'd like
stack_install could accept an additional attribute which takes a dict from package names to dicts from flag names to flag values, similar to Hazel's extra_flags attribute.

Additional context
Opening issue as suggested in #887 (comment).

[aherrmann]

This has been implemented in #1053.