(Add an option to?) write out a JSON file per repository

Comment from the code:

> write to a JSON file per repo
> the git history will have the longitudinal change wrt these scores
> ```python
> status_dir = self.basepath / 'status'
> status_dir.mkdir(parents=True, exist_ok=True)
> with (status_dir / (name.replace('/', '__') + '.json')).open('w') as f:
> json.dump(rstatus, f)
> ```

(where `rstatus` has the fields `full_name`, `size`, `stargazers_count`, `watchers_count`, `forks_count`, `open_issues_count`, `network_count`, `subscribers_count`, and `open_prs_count`, and where `self.basepath` is a directory path given on the command line, defaulting to `$PWD`)