bugfix for mdoc update with --dropns feature

There was a limitation with the prior release identified which happened when you ran mdoc update with multiple assemblies in the list. Previously, the "namespace drop" was assumed to be for all referenced types, however, that was an invalid assumption.

So this new version of mdoc introduces a new command line parameter: `--dropns-assemblies`. This is to be a comma separated list of assemblies that have dropped the assembly. This was required because otherwise there's no way of knowing which assembly to apply the changes to.

so a sample invocation would go like this: 
```
mdoc update assembly1 assembly2 --out=en -dropns --dropns-assemblies=assembly1
```

This pull request also contains an automated test for the plain-vanilla case of `-dropns`, and also to reproduce/test this new `--dropns-assemblies` functionality.
This was good feedback, I modified the code so that there's only one parameter, `dropns`, with a slight variation of your code, which worked perfectly:

			{ "dropns=",
			  "When processing assembly {ASSEMBLY}, strip off leading namespace {PREFIX}:\n" +
			  "  e.g. --dropns ASSEMBLY=PREFIX",
			  v => {
			    var parts = v.Split ('=');
			    if (parts.Length != 2) { Console.Error.WriteLine ("Invalid dropns input"); return; }
			    var assembly = Path.GetFileName (parts [0].Trim ());
			    var prefix = parts [1].Trim();
			    droppedAssemblies.Add (assembly);
			    droppedNamespace = prefix;
			} },

The ability to supply multiple assemblies really wasn't a core requirement ... we really just needed to specify one when I really sat and thought about it :)