Make a configuration management tool based on Nix to manage User Profiles.

This pull request is made to bootstrap a variant of NixOS, which is dedicated at managing the home directory of users.  This new project is called NixUP (Nix User Profile), and the goals are the following:

 - [✓] Generate an activation script.
 - [✗] Abstract for the creation of resource files. (similar to environment.etc)
 - [✗] Option to list installed packages. (similar to system.packages)
 - [✗] Command line tool to manage profiles generations and updates.  (similar to nixos-rebuild)
 - [✗] Command line tool to write / read option definition without using an editor.  (no legacy yet)
 - [✗] Mock compatibility modules to use some NixOS modules in NixUP.
 - [✗] Include NixUP modules as submodules of NixOS option `users.extraUser.<name?>`