Add ability to generate NuGet spec from the package manager console

[jchadwick]

Would be nice to have a quick way to move from a .nuspec-less NuGet.Packer project to creating a .nuspec file to customize the resulting package.

Would be nice to have a PowerShell command to do this from the NuGet Package Manager Console.

[jchadwick]

Or, maybe a Visual Studio extension (ala #12) would be a good way to handle this, too!