Add support for new-build

[hvr]

While looking at the poster-child example from the README, I noticed that you'd only need a small additional feature to help with new-build:

Specifically, the stack.yaml file mentions

packages:
- .
- upstream/snap-app

...moreover, the lts-3.15 "resolver" implies GHC 7.10.2

This combined would translate into the following cabal.project file:

with-compiler: ghc-7.10.2
packages: ./, upstream/snap-app

and together with the freeze file (which you'd have to redirect into the file cabal.project.freeze), cabal new-build is able to build the haskellnews project just fine.

[erikd]

I'm not using new-build yet, but I'd be happy to get a patch for this.