Expose method for generating a .nuspec

[Pxtl]

Summary of the new feature / enhancement

The .psd1 file contains all of the info needed to generate a .nuspec, which you're using internally. However, not all nuget hosts are fully compatible with PowerShellGet. Rather than try to target them perfectly, can you not just expose the internals to allow us to build the .nupkg ourselves?

Proposed technical implementation details (optional)

Extract the code for generating .nuspec in https://github.com/PowerShell/PowerShellGet/blob/f41ee9c9583ba26b982008d943a54eb73d4a68da/src/code/PublishPSResource.cs into its own cmdlet.

[partlythomas]

+1 It would be beneficial to allow the created nuspec file to persist rather than immediately remove it once the package is published/attempted published.