add support for fast-export authorfile feature

[mkvonarx]

When importing from Mercurial to Git with git-hg, I always get commiter/author email addresses of devnull@localhost. This is because I don't have email addresses in my Mercurial commit metadata.

fast-export supports a neat authorfile feature (-A --authors) that allows to manually map Mercurial user metadata to Git user metadata. It would be nice if git-hg would also support this fast-export feature.