Add contrib integrations to home-manager module

[xlambein]

This adds home-manager options to enable specific integrations from the lib-contrib. Currently, this includes bash, zsh, fish, vim, gtk, and the Nix wallpaper for Gnome.

The module's a bit bare, it could use some more documentation. I mostly wanna know whether you're interested to upstream this :]

[Misterio77]

Sorry for the wait! Overall everything looks good, I'll see if I can add some documentation before merging.

Thanks a lot for the contribution!

[bketelsen]

subscribing so we can add this enhancement to fleek this would be awesome.

[xlambein]

Would it be okay for you if I pushed to this PR to make these changes?

Of course! :] I'm totally in line with all those changes.

[no-mood]

Any update on this? Since I'm also interested, I could help if you are too.

[lpchaim]

I've been following this for a while and thought I'd chime in, I'd love to give the proposed changes a go or assist with this however is deemed helpful.

[eureka-cpu]

I think @Misterio77 is busy w school, etc, it's been over a year since his last review. If you want to see this implemented it may just be faster to make a fork of it..

[no-mood]

I don't mean to sound rude by saying that this feature is provided by Stylix (at the time I didn't know it). In that case, misterio's nix-colors compares to base16.nix, that's used in Stylix to import a theme and set it system wide. Thank you, Misterio, for all your tremendous work. You are the source of inspiration for so many users.