Add IPython extension support

[landeqiming666]

Summary

• add IPython extension entry points so %load_ext better_exceptions and %unload_ext better_exceptions work
• install an idempotent traceback hook that formats normal exceptions and delegates syntax errors and special IPython modes back to IPython
• document the IPython usage and add focused integration tests

Fixes #10.

Validation

• PYTHONPATH=. python3 test/test_ipython.py
• PYTHONPATH=. python3 test/test_logging.py
• python3 -m py_compile better_exceptions/init.py better_exceptions/integrations/ipython.py test/test_ipython.py
• git diff --check

[landeqiming666]

Hi, a brief follow-up on this PR. It remains clean and mergeable. The IPython extension entry points and load/unload behavior are covered without changing the existing default integration. Happy to adjust the extension API or tests.