Support for taking an (attrset, generator) pair

[uku3lig]

I've been using hjem a little bit and my main gripe with it is that it kind of cancels out the appeal of nix: when using HM, you can leverage the extensibility of the module system to add and remove things on the fly (eg. for having a different git email on a work and personal machine); however, hjem loses that extensibility since you're working with, well, strings.

My idea would span out this way:

# common.nix
hjem.users.uku.files.".gitconfig" = {
  generator = lib.generators.toGitINI;
  value = { ... };
};

# and then in work.nix, for nixosConfigurations.work
hjem.users.uku.files.".gitconfig".value.user.email = "uku@work.job";

I don't know if this idea is completely out of scope for hjem, but in my opinion it would be a nice middle ground between working with raw strings and having a massive per-program module system à la HM, since you're still only dealing with config files.

[NotAShelf]

Please note that Hjem is not a 1:1 Home-Manager replacement. We intend to keep the codebase small and streamlined, which is the reason we decided to opt out of a complete module system. Though, the file linking API is deliberately familiar with Home-Manager's. I also find it a little out of scope to try and handle complex value merges, though Hjem-Rum exists as an intricate module system to handle this, and it bridges the gap between HM and Hjem. I am happy to be delegating this to Hjem-Rum personally to keep Hjem itself simple. You might also build a little option system yourself, which would handle merges in-config.

[eclairevoyant]

Created #29, we'll explore feasibility later on. Do note that we don't plan to provide any generators, just the framework at most if we go with this.

[eclairevoyant]

implemented in #44