Allow controller generation to include areas

I'm trying to scaffold a controller in an area ie:

```
dotnet aspnet-codegenerator controller -m Order -dc DbContext -udl --referenceScriptLibraries --useDefaultLayout -outDir Areas/Admin/Controllers --controllerName OrdersController
```

It would be nice if I can pass an explicit `--area` or `-a` flag so that it will generate controllers (and related views) with the specified area (In this case "Admin"). This is important, because the default anchor tag helper doesn't reuse the current area, so if I generate and try to move to it to an area, I have to manually edit all the files and add in `