Add an option to choose the format for the author name

[till-varoquaux]

I work with two people with the same last name and can't tell apart in git-blame. Having the ability to choose to display either first name, last name, full name or email address would be great.

[javawizard]

Just wrote up #112.

(Showing email addresses/first names only/etc. would be a neat idea. Don't care enough myself to implement it, though.)

[alexcorre]

@till-varoquaux happy to entertain a PR for this change. Can follow #112 lead.