Add a feature to NuGetRepack to block generating release packages for certain pre-release NuGet packages

Roslyn-analyzers repo currently has an experimental NuGet package for dataflow based analyzers `Microsoft.CodeQuality.Analyzers.Exp.nupkg`. We want to ship pre-release version of this package to NuGet to allow dogfooding, but never want to ship a release package (once we are comfortable with the DFA based rules to make it release, these rules will be moved to other existing analyzer packages and we will delete `Microsoft.CodeQuality.Analyzers.Exp`). It would be good if NuGetRepack can support blocking generation of release packages for such experimental packages - essentially address https://github.com/dotnet/roslyn-analyzers/pull/1622#issuecomment-369440112.