The bulk login endpoint is here! You can request up to 100 users at a time.
Ex: https://api.twitch.tv/kraken/users?api_version=5&client_id=&login=dallas,3v,BarryCarlyon
CC: @SeeingBlue, @BarryCarlyon, @moocat, @Stricted, @freaktechnik, @CBenni, and probably more.