Add IPython traceback integration

[apples-kksk]

Fixes #10

Summary

• add an IPython extension that routes IPython tracebacks through better_exceptions
• let users load it with %load_ext better_exceptions
• make better_exceptions.hook() install the IPython hook when an active shell exists
• add regression coverage with a fake IPython shell and document usage

Verification

• PYTHONPATH=. python3 test/test_ipython.py
• python3 -m py_compile better_exceptions/init.py better_exceptions/integrations/ipython.py test/test_ipython.py
• FORCE_COLOR=0 TERM=dumb PYTHONPATH=. python3 test/test_logging.py
• Temporary real IPython smoke test: %load_ext better_exceptions / %unload_ext better_exceptions
• git diff --check

Note: PYTHON=python3 ./test_all.sh could not complete because /usr/bin/expect was missing for test/test_interactive.sh; the failure occurs before the new IPython test is reached.