Provide a release summary

[jaraco]

Let's find a way to provide a release summary, showing the date it was released and a summary of the changes. It should degrade gracefully to just regular commit messages, but it should also have features like:

• detect what is the scope of the next release (bugfix, feature, removal)
• highlight curated user-facing messages
• summarize the changes
• link to the diff since the last release in GitHub