Ported to .Net Standard 1.3

This version mirrors the 1.2.0 release on NuGet:
https://www.nuget.org/packages/SteGriff.Paprika.Net/1.2.0

• Much improved CLI and error handling
• More help when the app opens
• New commands including a grammar validator
• For help, type //? while running the interactive console.

This is a good working version of Paprika. I'm planning a big refactor which may end up good or bad, but this version is just fine!