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>

README.md

@@ -229,6 +229,9 @@ require('nuwiki').setup({
       template_default = 'default',
       template_ext     = '.tpl',
       css_name         = 'style.css',
+      custom_wiki2html = '',             -- external converter; empty = built-in renderer
+      custom_wiki2html_args = '',        -- extra args appended to the converter call
+      base_url         = '',             -- public URL prefix for RSS item links
       auto_export      = false,          -- export on save
       auto_toc         = false,          -- auto-refresh TOC on save
       auto_generate_links = false,       -- regen Generated Links on save
@@ -358,6 +361,9 @@ Each is a boolean. All default to `true` except `mouse`.
 | `css_name` | string | `'style.css'` | stylesheet filename |
 | `auto_export` | bool | `false` | `true` \| `false` (export on save) |
 | `html_filename_parameterization` | bool | `false` | `true` \| `false` |
+| `custom_wiki2html` | string | `''` | external converter command; empty = 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 |
+| `custom_wiki2html_args` | string | `''` | extra args appended to the `custom_wiki2html` call |
+| `base_url` | string | `''` | public URL prefix; when set, diary RSS item links become `<base_url><diary_rel_path>/<date>.html` instead of `file://` |
 | `exclude_files` | list | `{}` | glob patterns |
 | `color_dic` | table | `{}` | `{ name = 'color', … }` |