tmux nixos module: add nixos program module for tmux

This basic module allows you to specify the tmux configuration.

As great as tmux is, some of the defaults are pretty awful, so having a
way to specify the config really helps.

nixos/modules/module-list.nix

@@ -77,6 +77,7 @@
   ./programs/shell.nix
   ./programs/ssh.nix
   ./programs/ssmtp.nix
+  ./programs/tmux.nix
   ./programs/venus.nix
   ./programs/wvdial.nix
   ./programs/xfs_quota.nix

nixos/modules/programs/tmux.nix

@@ -0,0 +1,35 @@
+{ config, pkgs, lib, ... }:
+
+let
+  inherit (lib) mkOption mkEnableOption mkIf mkMerge types;
+
+  cfg = config.programs.tmux;
+
+in
+{
+  ###### interface
+
+  options = {
+    programs.tmux = {
+
+      enable = mkEnableOption "<command>tmux</command> - a <command>screen</command> replacement.";
+
+      tmuxconf = mkOption {
+        default = "";
+        description = ''
+          The contents of /etc/tmux.conf
+        '';
+        type = types.lines;
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+    environment = {
+      systemPackages = [ pkgs.tmux ];
+      etc."tmux.conf".text = cfg.tmuxconf;
+    };
+  };
+}