add support for nix

This resolves #143.

Author: jpotier

after/syntax/nix.vim

@@ -0,0 +1,2 @@
+" https://github.com/LnL7/vim-nix
+call css_color#init('hex', 'none', 'nixComment,nixString,nixSimpleString')

tests/example.nix

@@ -0,0 +1,69 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.colorscheme.nord;
+  hexColor = types.str // {
+    check = (x: hasPrefix "#" x && builtins.stringLength x == 7);
+    description = "hexadecimal color";
+  };
+  mkHexColorOption = name: default:
+    mkOption {
+      type = hexColor;
+      default = default;
+      description = name;
+    };
+in {
+  ## POLAR NIGHT
+  # The origin color or the Polar Night palette.
+  nord0  = mkHexColorOption "nord0"  "#2E3440";
+
+  # A brighter shade color based on nord0.
+  nord1  = mkHexColorOption "nord1"  "#3B4252";
+
+  # An even more brighter shade color of nord0.
+  nord2  = mkHexColorOption "nord2"  "#434C5E";
+
+  # The brightest shade color based on nord0.
+  nord3  = mkHexColorOption "nord3"  "#4C566A";
+
+  ## SNOW STORM
+  # The origin color or the Snow Storm palette.
+  nord4  = mkHexColorOption "nord4"  "#D8DEE9";
+
+  # A brighter shade color of nord4.
+  nord5  = mkHexColorOption "nord5"  "#E5E9F0";
+
+  # The brightest shade color based on nord4.
+  nord6  = mkHexColorOption "nord6"  "#ECEFF4";
+
+  ## FROST
+  # A calm and highly contrasted color reminiscent of frozen polar water.
+  nord7  = mkHexColorOption "nord7"  "#8FBCBB";
+
+  # The bright and shiny primary accent color reminiscent of pure and clear ice.
+  nord8  = mkHexColorOption "nord8"  "#88C0D0";
+
+  # A more darkened and less saturated color reminiscent of arctic waters.
+  nord9  = mkHexColorOption "nord9"  "#81A1C1";
+
+  # A dark and intensive color reminiscent of the deep arctic ocean.
+  nord10 = mkHexColorOption "nord10" "#5E81AC";
+
+  ## AURORA
+  # RED
+  nord11 = mkHexColorOption "nord11" "#BF616A";
+
+  # ORANGE
+  nord12 = mkHexColorOption "nord12" "#D08770";
+
+  # YELLOW
+  nord13 = mkHexColorOption "nord13" "#EBCB8B";
+
+  # GREEN
+  nord14 = mkHexColorOption "nord14" "#A3BE8C";
+
+  # PURPLE
+  nord15 = mkHexColorOption "nord15" "#B48EAD";
+}