first nuget release

  2020/05/06
  1. extend target frameworks to netcoreapp3.0;netcoreapp3.1;net461;net462;net47;net471;net472;net48
  2. add pre-build event to do dotnet restore. Issue description at https://stackoverflow.com/questions/48440223/assets-file-project-assets-json-not-found-run-a-nuget-package-restore