neovim: add follow_url_func option

outputs/pkgs/ook-vim/modules/plugins/obsidian/obsidian.nix

@@ -86,6 +86,17 @@ in {
           Customize how a note ID is generated given an optional title
         '';
       };
+      follow_url_func = mkOption {
+        type = nullOr luaInline;
+        default =
+          mkLuaInline
+          # lua
+          ''
+            function(url)
+              vim.fn.jobstart({"xdg-open", url})
+            end
+          '';
+      };
       ui = {
         enable = mkOption {
           type = nullOr bool;