Commandline arguments to pass along to NuGet

I've added a NuGetArguments property to the CreateOctoPackPackage task, with which you can specify extra NuGet arguments that OctoPack should use when it calls NuGet Pack.

I've used it to pass along -NoDefaultExcludes, so that folders and files starting with a dot are included.

Please tell me if there's anything I should do/add to make this pull request more mergable :)