Add an overlays option

[srid]

While people can use basePackages = pkgs.haskellPackages.extend ... to already do this, let's add an overlays option anyway for efficiency purposes (see #162 (comment)).

However, unlike packages and settings the overlays type should probably be made un-mergeable when an user imports a haskell-flake project module, since the order is not something the user can specify.

Ultimately, most users would just use packages and settings - whereas the small subset who would want to stay close to overlays can still specify them, whilst not being able to use the haskell-flake shareable module system.

[srid]

#277