Add support for .txt files in extract and summarize

[cedsic]

Currently, PyAI Extract & Summarize supports PDF, CSV/XLSX, and image files.
It would be useful to also support plain text (.txt) files.

Tasks:

1. Update extract_file() to handle .txt files.
2. Update summarize_file() to handle .txt files.
3. Add a test case in test_core.py using a sample .txt file.