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!