Create VSO vNext Build Task for calling GitLink

[clairernovotny]

VSO's vNext build agents supports building from GitHub and any other external Git repo. GitLink can/should be packaged up as a Build Step for use there more easily.

See GitVersion for an example: https://github.com/GitTools/GitVersion/tree/master/src/GitVersionTfsTask

[GeertvanHorrik]

Thanks, we will consider adding a task.