Improve release procedure: generate changelog using script

[snomos]

The idea is to semi-automatically generate or update changelogs. See e.g.

• https://stackoverflow.com/questions/40865597/generate-changelog-from-commit-and-tag
• https://stackoverflow.com/questions/3523534/what-are-some-good-ways-to-manage-a-changelog-using-git
• https://stackoverflow.com/questions/16619753/automatically-write-changelog-to-changelog-md-in-github

We might consider having separate changelogs for analysers (default), spellers, grammar checkers, etc, based on paths of modified files. The different changelogs would correspond to different release tags. The present tag system only supports speller releases.