Add snapshots attribute

[rvl]

This provides stackage LTS package sets that you can just use without needing mkStackPkgSet.

This will be useful together with ghcWithPackages and ghcWithHoogle (PR #150).

[angerman]

So this will give us bare lts snapshots? Wouldn't we want to allow to add a set of packages to them?

I'm thinking someone might want lts-X.Y + [ pkg-A.B.C.D ... ]?

[rvl]

The idea is that you could use these snapshots more or less like haskellPackages in nixpkgs.

Wouldn't we want to allow to add a set of packages to them?

You would need to use mkStackPkgSet for this. Haskell.nix offers no way of extending a package set after it's been created.

[angerman]

I was wondering if we could make

haskellPackages = self.snapshots."lts-13.18";

take a parameter, and then just inject the packages. But That would bring in all kinds of nasty dependency solving issues.

[angerman]

@rvl, I think we should merge this if with the added comment as per the review above.