Support flags directive in stack.yaml when generating packages

[domenkozar]

Example: https://github.com/input-output-hk/cardano-sl/blob/master/stack.yaml#L5

Most probably depends on #5

[jmitchell]

We also need to support the ghc-options directive. Example: https://github.com/input-output-hk/cardano-sl/blob/423a1e5dfd87a693e2046dcabc767a001594d1eb/stack.yaml#L141

[MMesch]

another project that requires this feature:

https://github.com/AccelerateHS/accelerate-fft/blob/master/accelerate-fft.cabal

[cdepillabout]

Has this been fixed by #148?

[domenkozar]

Still need to support ghc-options, although that should be a new issue probably.