neovim: mark plugins as optional (possibly) (#1559)

can be loaded with packadd!

modules/programs/neovim.nix

@@ -24,15 +24,20 @@ let
 
   pluginWithConfigType = types.submodule {
     options = {
-      plugin = mkOption {
-        type = types.package;
-        description = "vim plugin";
-      };
       config = mkOption {
         type = types.lines;
         description = "vimscript for this plugin to be placed in init.vim";
         default = "";
       };
+
+      optional = mkEnableOption "optional" // {
+        description = "Don't load by default (load with :packadd)";
+      };
+
+      plugin = mkOption {
+        type = types.package;
+        description = "vim plugin";
+      };
     };
   };
 
@@ -49,8 +54,15 @@ let
     || (lib.filter (hasAttr "config") cfg.plugins) != [ ]) {
       customRC = cfg.extraConfig
         + pkgs.lib.concatMapStrings pluginConfig cfg.plugins;
-    } // optionalAttrs (cfg.plugins != [ ]) {
-      packages.home-manager.start = map (x: x.plugin or x) cfg.plugins;
+
+      packages.home-manager = {
+        start = filter (f: f != null) (map (x:
+          if x ? plugin && x.optional == true then null else (x.plugin or x))
+          cfg.plugins);
+        opt = filter (f: f != null)
+          (map (x: if x ? plugin && x.optional == true then x.plugin else null)
+            cfg.plugins);
+      };
     };
   extraMakeWrapperArgs = lib.optionalString (cfg.extraPackages != [ ])
     ''--prefix PATH : "${lib.makeBinPath cfg.extraPackages}"'';