Support cabal.project files

[dbaynard]

Cabal now has cabal.project files, which overlap with some of the features of stack.yaml files. It would be good for stack to be able to convert between stack.yaml and cabal.project files.

• stack2cabal can do one direction.
• This article by @fommil discusses further details.

[snoyberg]

Would you be OK if I moved this to the wishlist?

[dbaynard]

Absolutely — it seems like exactly the right place for it. Thanks!

[snoyberg]

Awesome, added.