Add support for mailmap configuration

Following the spirit of improving GitLab for large open source project; GitLab could add support for .mailmap configuration.

The idea would be that this way any open source contributor that is using more than 1 email address in git can claim his/her open source contributions just by adding this standard git configuration file to the project.

This is something that happens mostly when the developer is using at least 2 hosts where each one has a different git configuration (personal/professional).

Probably once this file is added/touched gitlab should trigger some kind of background processing to assign the right contributions to the right user as those contributions would be changing owner.

Documentation about the feature: https://git-scm.com/docs/git-shortlog#_mapping_authors

For reference of how this impacts open source contributors you can listen to the bikeshed podcast: http://bikeshed.fm/35