Add option to merge namespaces

[eduherminio]

                [Option(shortName: 'm', longName: "merge-namespaces", Required = false, Default = false, HelpText = @"
        Groups the output code by namespace.")]
                public bool MergeNamespaces { get; set; }

Implement 'properly' using Roslyn.

[ardalis]

Namespace support would be excellent, thanks.

[eduherminio]

I better start with #92 😅