Code cleanup

[maxmouchet]

• Remove passenger store and associated CLI flag (unused feature, may be re-implemented later if needed)
• Separate offline/online tests (use pytest markers ? http://doc.pytest.org/en/latest/example/markers.html)
• Separate CLI repo (locomotive-cli ?)
• Merge mypy and isort config. to pyproject.toml when supported
• abstract -> base, multiple inheritance for features supported by the clients
• More tests, increase coverage
• Change logo (tunnel), update README.md (Julia style)
• Multithreaded clients
• Proper date formatting (babel, pendulum, ?)