v0.5.7: Counters are Multisets

What's Changed

• Implement is_subset and is_superset tests by @mjpieters in #27
• Implement bitwise and and or assignments by @mjpieters in #28
• Fix spelling error in doc header by @mjpieters in #29
• Multiset bag documentation by @coriolinus in #31

New Contributors

• @mjpieters made their first contribution in #27

Full Changelog: v0.5.6...v0.5.7