Create script for listing all contributors of a release

It's a good habit in our release announcements to list all the contributors. Gathering the names and GitHub handles is rathre tedious, though.

Instead, let's have a script which automates that task and emits a C&P ready MarkDown snippet. It should collect the names (linked to GitHub profile) of all contributors since the last release to the different repos (core, incubator, docker-images, vitess).

Ideally, this runs as part of the release job, so the author of the announcement can simply go there and copy the text. But it also should be runnable locally.