Support for mailmap

Git has awesome `.mailmap` configuration allowing you to describe which emails are the same user. There's more documentation here: https://git-scm.com/docs/git-check-mailmap

Example report from gitstats:

```
█ 589 | foo@example.com
███ 2882 | foo@gmail.com
1 | foo@Foo.local
```

With this `.mailmap` document:
```
# Mapping historically incorrect Git author names and emails
John Doe

```

git would actually use only the left aliases and combine results in `git log`, `git shortlog` and others.

It would be great if `gitstats` could do the same using `git` plumbing commands and make use of the `.mailmap` configuration in the repository.

Thanks!

Start by tracing gitstats's report-generation entry point and review Git's git-check-mailmap documentation. Determine where author names and emails are aggregated, then verify that repository .mailmap mappings combine aliases in the generated statistics, including the examples described in this issue.

Written by the indexing model from the issue text.