pubs: add module

Note, the pubs configuration file uses ConfigObj syntax, which is
similar to the INI files syntax but with extra functionalities like
nested sections. This prevents it from using Nix's INI format
generator. Here is an example of pubs configuration that cannot be
generated using Nix's INI format generator:

    [plugins]

    [[git]]
    manual=False

For this reason, we opted for a stringly-typed configuration since the
use of a structured `settings` option would require a custom parser.

modules/programs/pubs.nix

@@ -0,0 +1,60 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.pubs;
+
+in {
+  meta.maintainers = [ hm.maintainers.loicreynier ];
+
+  options.programs.pubs = {
+    enable = mkEnableOption "pubs";
+
+    package = mkOption {
+      type = types.package;
+      default = pkgs.pubs;
+      defaultText = literalExpression "pkgs.pubs";
+      description = "The package to use for the pubs script.";
+    };
+
+    extraConfig = mkOption {
+      type = types.lines;
+      default = "";
+      example = literalExpression ''
+        '''
+        [main]
+        pubsdir = ''${config.home.homeDirectory}/.pubs
+        docsdir = ''${config.home.homeDirectory}/.pubs/doc
+        doc_add = link
+        open_cmd = xdg-open
+
+        [plugins]
+        active = git,alias
+
+        [[alias]]
+
+        [[[la]]]
+        command = list -a
+        description = lists papers in lexicographic order
+
+        [[git]]
+        quiet = True
+        manual = False
+        force_color = False
+        ''''';
+      description = ''
+        Configuration using syntax written to
+        <filename>$HOME/.pubsrc</filename>.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    home.packages = [ cfg.package ];
+
+    home.file.".pubsrc" =
+      mkIf (cfg.extraConfig != "") { text = cfg.extraConfig; };
+  };
+}