Markdown unknown tags

[skatkov]

I want to be able to control how html to markdown conversion works with this gem. This PR introduces a configurable attribute that will be passed to ReverseMarkdown gem

[Copilot]

Pull request overview

Adds a new RDoc option to control how reverse_markdown handles unknown HTML tags when generating Markdown, with corresponding CLI/config documentation and tests to validate behavior and error handling.

Changes:

• Introduces markdown_unknown_tags as an RDoc::Options setting (plus CLI flag --markdown-unknown-tags) and validates accepted modes.
• Passes the configured unknown_tags mode through to ReverseMarkdown.convert during markdown generation.
• Adds coverage for default behavior, each supported mode, CLI parsing, .rdoc_options hash loading, and YAML serialization cases; documents usage in the README and changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/test_markdown_helpers.rb	Adds tests and helper utilities to cover the new markdown_unknown_tags option across default/CLI/YAML behaviors.
README.md	Documents the new option, accepted values, and .rdoc_options configuration.
lib/rdoc/generator/markdown.rb	Implements the option plumbing (RDoc::Options extension, CLI registration, validation) and wires unknown_tags: into ReverseMarkdown.convert.
CHANGELOG.md	Records the addition of the new markdown_unknown_tags option.

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