Add Wikipedia extractor to extract article content as Markdown

[michaelos443]

Description

This PR adds a new extractor for Wikipedia that allows users to extract and save article content in Markdown format. This is useful for offline reading, research, or creating local archives of Wikipedia articles.

Features

• Extract text content from Wikipedia articles
• Save content as Markdown files
• Preserve headings and paragraph structure
• Clean up unwanted elements (references, edit links, etc.)

Changes

• Added new wikipedia.py extractor module
• Updated SITES dictionary in common.py
• Updated extractors/init.py to include the new module
• Added BeautifulSoup4 as a dependency in requirements.txt and setup.py
• Updated README.md with documentation for the new feature

Example Usage

$ you-get https://en.wikipedia.org/wiki/Free_software
Site:       Wikipedia
Title:      Free software
Type:       Markdown
Size:       0.12 MiB (123456 Bytes)

Wikipedia article saved to: Free software.md

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Pull Request opened by Augment Code with guidance from the PR author

[michaelos443]

augment review