Ability to reference common Maven repositories by constant instead of manually

[UnknownJoe796]

It would be nice to include things like JCenter and JitPack extremely easily. This could be done super easily like this:

import org.apache.maven.model.Model
...
Model().apply {
  repositories = listOf(Repositories.jcenter, Repositories.jitpack)
}

I'd recommend the creation of:

object Repositories {
  val jcenter: Repository = ...
  val jitpack: Repository = ...
}

This is really easy to do, but this is a first good task for somebody else to take who wants to get familiar with the project.

Don't forget to create a unit test that uses it!