Add home manager module to flake outputs

[akiekintveld]

It looks like the NixOS module was made available for flakes, but the home manager module was not. This change adds it to the flake outputs and updates the documentation to show how to use it.

[msteen]

Does the home manager module work for you? No issues like #30?

[ghost]

I added this to my personal config yesterday and didn't see any issues like that when I rebuilt three machines.

[ghost]

Closing in favor of #34, I think it makes more sense to have this under nixosModules.