Replace the custom configuration mechanisms (for processors, generators, etc...) with Microsoft.Extensions.Configuration and Microsoft.Extensions.Options.

I'm not sure if I should throw out the connection string handling for machine.config, app.config, or web.config. I'd like to, but I guess the that it still makes sense to support this scenario for .NET Framework builds, because there are still many ASP.NET (non-core) web sites out in the wild.

@escarls What is your opinion?