Add --output option alias

[StantonMatt]

Summary

• Add --output <file> as a conventional alias for the existing --savepath behavior.
• Add --output - as a conventional alias for the existing --print stdout behavior.
• Cover both aliases with focused init() tests.

This keeps the default behavior unchanged while addressing the discoverability suggestion from #90.

Tests

• .venv/bin/python -m unittest tests.test_pipreqs.TestPipreqs.test_init_output_path tests.test_pipreqs.TestPipreqs.test_init_output_dash
• .venv/bin/python -m unittest discover
• .venv/bin/flake8 pipreqs tests
• git diff --check
• review-fix-loop: no actionable correctness issues

[bubaizhanshen]

I checked this locally on Python 3.10.

The alias behavior works in a direct probe with local package lookup mocked:

• --output out.txt writes the generated requirements to that file
• --output - writes to stdout and does not create the default requirements.txt

Static checks passed:

python -m py_compile pipreqs/pipreqs.py tests/test_pipreqs.py
python -m flake8 pipreqs tests
git diff --check upstream/master...HEAD

The focused unit command from the PR body waits on a live PyPI request in this environment through test_init_output_path, so I did not use it as the main signal here.