Make it easier to navigate between projects in the output

[JelleZijlstra]

When I look at a diff-shades diff, I often want to be able to look at changes in separate projects, because they can give a better sense of how the change affects different code styles. (In the PR, I just looked at, I already scrolled through bazillions of changes to Django, but I want to find the next project.)

Maybe the diff can have a table of contents of some sort that links to the individual projects with changes. Or there can just be a list of projects with changes, so I can search for say "mypy:" to get to the next project.