Add a CHANGELOG/release notes generator

Looking through github logs is tedious. We could have a tool which would read the logs back to a particular point (usually last major or minor release) and use that to:

Extract RELNOTES sections from them and print them.
If they fix issues, find those issues and add them to an "Issues Fixed" section
Extract authors and use that to build a thank-you section.

The bazel release tool does something similar, but it is in bash, which limits portability.