Allow specifying local cabal options

[RyanGlScott]

Just as there is currently a --local-ghc-options flag for adding ghc-options to the autogenerated cabal.project file, it would be convenient to have something similar for flags. I currently have to add this by hand to free's .travis.yml, for instance:

https://github.com/ekmett/free/blob/26b5a9b947969ff85e419443ae11e26319693a00/.travis.yml#L106-L108

[RyanGlScott]

Or perhaps we need a more general mechanism for adding stanzas to a package field in the auto-generated cabal.project file. (For instance, I just found myself needing to generate library-coverage: true.)

[RyanGlScott]

I think what might be nicer is to allow users to write stub files (lousy name proposal: cabal.project.stub) with contents like:

package my-package
  library-coverage: true

And have haskell-ci append that to the end of its auto-generated cabal.project files. This way, we wouldn't need an abundance of flags to support every possible Cabal flag.

[RyanGlScott]

#157 added constraint sets via .haskell-ci files. However, it's unclear to me what the relationship between that and this idea is (for instance, I'm not sure if I can use constraint sets to achieve what I want here).

[phadej]

This will be resolved once next Cabal version is released. Hopefully soon.

[RyanGlScott]

This was addressed in #129 with the addition of raw-project stanzas, which let you do things like this:

raw-project
  package my-package
    library-coverage: true