feat(config): custom_wiki2html external converter + base_url for RSS (P2)

Add three per-wiki HtmlConfig keys mirroring vimwiki globals:

- custom_wiki2html / custom_wiki2html_args: when set, export shells out
  to the external converter via `sh -c` with vimwiki's exact arg order
  (<cmd> <force> <syntax> <ext> <out_dir> <in_file> <css> <tpl_path>
  <tpl_default> <tpl_ext> <root_path> <args>, `-` for empty optionals)
  instead of the built-in renderer.
- base_url: when set, diary RSS item + channel links become
  <base_url><diary_rel_path>/<date>.html instead of file:// URIs.

write_page() gains a `force` flag threaded through export_current (false),
export_all (its own flag) and the did_save auto-export path (false).

Tests: write_page_invokes_custom_wiki2html_converter,
write_rss_uses_base_url_for_public_links (html_export.rs), config
round-trip + defaults (index_and_config.rs). Docs: README, doc/nuwiki.txt,
lua config comment, vimwiki-gap.md item ticked.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lua/nuwiki/config.lua

@@ -26,7 +26,8 @@ M.defaults = {
   --   html_path, template_path, template_default, template_ext,
   --   template_date_format, css_name, auto_export, auto_toc,
   --   auto_generate_links, auto_generate_tags, auto_diary_index,
-  --   html_filename_parameterization, exclude_files, color_dic
+  --   html_filename_parameterization, exclude_files, color_dic,
+  --   custom_wiki2html, custom_wiki2html_args, base_url,
   --   toc_header, toc_header_level, links_header, links_header_level,
   --   tags_header, tags_header_level,
   --   listsyms, listsym_rejected, listsyms_propagate, list_margin,