feat: add EML email file converter

[mvanhorn]

Summary

Adds support for converting RFC 822 .eml files to Markdown using Python's built-in email module. Zero new dependencies. Follows the OutlookMsgConverter pattern exactly.

Why this matters

markitdown handles Outlook .msg files but not standard .eml files (#89). EML is the universal email format used by Thunderbird, Apple Mail, Gmail exports, and email forensic tools. Anyone processing email archives for LLM analysis needs this.

Changes

• _eml_converter.py: New converter handling message/rfc822 MIME type and .eml extension
• Multipart MIME support: prefers text/plain, falls back to text/html with tag stripping
• Extracts From, To, Cc, Subject, Date headers
• Registered in _markitdown.py alongside OutlookMsgConverter
• Two test vectors added: plain text email and multipart (text + HTML)

Testing

Both test vectors pass test_guess_stream_info and test_convert_local:

tests/test_module_vectors.py::test_guess_stream_info[test_vector5] PASSED
tests/test_module_vectors.py::test_guess_stream_info[test_vector6] PASSED
tests/test_module_vectors.py::test_convert_local[test_vector5] PASSED
tests/test_module_vectors.py::test_convert_local[test_vector6] PASSED

Relates to #89

This contribution was developed with AI assistance (Claude Code).