Add option to hide all unexported structs, interfaces and functions

[keep94]

I tried using the -hide-private-members option. This option hides all the private members of each struct or interface, but I still see all the private (unexported) structs without their private members in my UML diagram. As a client of the package I am diagramming, I am only interested in the exported structs and interfaces. Therefore, I'd like to hide all unexported structs / interfaces / or functions entirely.