make github username explcit

Using a tributors file to track BIDS contributors.

https://github.com/bids-standard/bids-specification/blob/master/.tributors

Several of them do not have a a github account.

Therefore I have been using a key `name_familyname` in the file.

But now I have no explicit way of knowing if that person has a github account.

Added it for some people: https://github.com/bids-standard/bids-specification/blob/790c0fbf72c01c0723f9afe3486cc154e0669d07/.tributors#L369

Is there an official way to handle that explicitly in tributors files?