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>

doc/nuwiki.txt

@@ -194,6 +194,16 @@ match upstream vimwiki.
 `template_default`       `'default'`
 `template_ext`           `'.tpl'`
 `css_name`               `'style.css'`
+`custom_wiki2html`       `''` — external converter command. When set, export
+                         shells out to it instead of the built-in renderer,
+                         called as: <cmd> <force> <syntax> <ext> <out_dir>
+                         <in_file> <css> <tpl_path> <tpl_default> <tpl_ext>
+                         <root_path> <args> (`-` for empty optionals), matching
+                         vimwiki's `g:vimwiki_custom_wiki2html` contract.
+`custom_wiki2html_args`  `''` — extra args appended to the converter call.
+`base_url`               `''` — public URL prefix. When set, diary RSS item
+                         links become <base_url><diary_rel_path>/<date>.html
+                         instead of local `file://` paths.
 `auto_export`            `false` — export to HTML on save.
 `auto_toc`               `false` — refresh TOC on save.
 `auto_generate_links`    `false` — regen an existing Generated Links section