Build for Scala 2.12

[llinder]

This looks like it might take some work. At very least Argot needs to be replaced with a maintained CLI argument parser that is published for 2.12.

[comeara]

Hi @llinder. Yes, I agree this is needed. I had a quick look this weekend and determined that I will also need to upgrade my ScalaTest version to 3.x in order to build Pillar for Scala 2.12.

I will work on the ScalaTest upgrade and finding a replacement for Argot and keep you updated as to how the is coming.

[Xeli]

At first I thought Pillar didn't support scala 2.12. But it seems like it does but we have to use pillar-core now.

https://mvnrepository.com/artifact/com.chrisomeara/pillar-core_2.12/3.0.0

@comeara perhaps it would be useful to refer to pillar-core in the readme on this repo?

[comeara]

@Xeli Nice investigation. I am in the middle of extracting pillar-core as a separate artifact but haven't released the next version yet. I will update the docs in the next release.