Add support for github and gitlab avatars

[timvink]

Design & implement a clean solution to support user avatars.

As a start, a detailed analysis has been done here:
squidfunk/mkdocs-material#1403

We can discuss how to implement a clean design here

[timvink]

There is now the excellent https://github.com/ojacques/mkdocs-git-committers-plugin-2 repository that deals with the github API to retrieve user avatar information. That means this project can keep focus on extracting information from the local git repo.

You can see how to set it up with mkdocs-material here: https://squidfunk.github.io/mkdocs-material/setup/adding-a-git-repository/#document-contributors

This is what it looks like: