Load argument defaults from `.githubactivity.json` file

[manics]

In most JupyterHub repos we use different defaults, e.g. we using heading-level 3: https://jupyterhub-team-compass.readthedocs.io/en/latest/practices/releases.html

This is an experimental PR to introduce a .githubactivity.json configuration file that could contain the defaults for the repository, minimising the number of arguments that are needed. In future this will also make it easier to customise the labels or categories used instead of github-activity having to cater for everyone. Since --since already defaults to the most recent tag this potentially means no arguments would be needed when running github-activity in most cases. What do you think?

To distinguish between default argparse arguments and CLI flags being explicitly set to the default value all argparse defaults are now set to None, with the actual defaults being set in load_config_and_defaults