Make nix-modules available when defining user environments.

A new user specific configuration file is introduced.
It is located in "~/.nixuser/configuration.nix".
The idea is to have a declarative approach for defining the user environment
at some point. The configuration file acts as the input of a module evaluation
system, similar to "/etc/nixos/configuration.nix" for the NixOS configuration.

A new file "nixos/modules/nixuser-module-list.nix" is added.
The file lists modules that are made available both for system configuration
and for user configuration.
The file is imported by "nixos/modules/module-list.nix" and is used in
"pkgs/top-level/all-packages.nix".
Add infrastruture: vimProfile

VimProfile allows to wrap vim with different configurations.
A configuration is composed by enabling and customizing pre-defined modules.
A module is a set of vim plugins and vimrc statements that are tightly linked.
Configurations are hierarchical and can inherit from each other.
The base of each hierarchy of configurations is a distingued default configuration.
Enabled vim configurations appear as normal package in nix, and have to be
installed accordingly.

The vimProfiles are declared either for system wide installations in
/etc/nixos/configuration.nix, or for each user in ~/.nixuser/configuration.nix.

An example for a vim configuration, here the content of ~/.nixuser/configuration.nix:

{ config, pkgs, ... }:
with pkgs;
{
  programs.vim =
    { enable = true;
      leader = " ";
      general.enable = true;
      statusline.enable = true;
      profiles =
        {  # Haskell configuration
          hvim =
            { enable = true;
              exec = { package = pkgs.vim_configurable; bin = "bin/vim"; };
              search.enable = true;
              # leader can be overwritten independently for each module
              search.leader = "|";
              text.enable = true;
              sidebar.enable = true;
              # overwrite default key binding
              sidebar.toggleUndo = "<Leader>uu";
              completion.enable = true;
              haskell.enable = true;
            };
          # graphical Haskell configuration
          hqvim =
            { enable = true;
              # set the name to call the wrapped vim
              name = "Hvim";
              parent = "hvim";
              exec = { package = pkgs.qvim; bin = "bin/qvim"; };
              # just as an example that inherited modules can also be disabled
              search.enable = false;
            };
          # same configuration as for default, but with qvim
          qvim =
            { # don't make that profile installable
              enable = false;
              exec = { package = pkgs.qvim; bin = "bin/qvim"; };
              # if no name attribute is given, the attribute name
              # of the configuration is taken: "qvim"
            };
          # alias declaration for qvim
          gvim =
            { enable = true;
              # inherit from a configuration by its attribute name
              # you can inherit from a disabled configuration as well
              parent = "qvim";
              # add statement to the generated vimrc; it is inserted
              # before any module statement
              vimrc = ''
                " Leader key timeout
                set tm=2000
                '';
            };
        };
    };
}

The declaration above defines the following packages in nix:
"vimProfile.hvim", "vimProfile.hqvim" and "vimProfile.gvim".
When the default profile is enabled, it is automatically installed with
the package "vim".
An un-wrapped vim package is always available with "vimPlain".