Add markdown() method to Response class

[supercoder-dev]

This implements the feature requested in issue #339. I've added a new markdown() method to the Response class. This method extracts markdown text from the response content, making it easier to work with LLM-friendly formats. used BeautifulSoup and html2text libraries, which are now optional dependencies.

[perkfly]

Cool, thanks.

We need to add these dependencies as an optional dependency group in pyproject.toml and add unit tests for this method.

Besides, I would like to add readability-lxml and lxml as another extract engine.

With all due respect, I think this comment sounds like a LLM-powered bot.