Target .NET Standard, add C# source generator and .NET Core tool

[kentcb]

This is a big one:

• I've targeted .NET Standard (yes, now the name "PCLMock" doesn't make quite as much sense, but buggered if I can be bothered renaming the project)
• I've dropped the T4 template support
• I've added support for generating mocks via a C# Source Generator
• I've switched the console support over to a .NET Core tool
• I've fixed and improved a whole bunch of issues with the code generator itself
• Using SDK-style projects now
• Improved the build/CI scripting by leveraging built-in packaging support and jumping from Fake to Cake

Fixes #31.