Add a way to access the returned value of the original method

[ascott18]

I need a way to receive the original return value of the underlying method, and then either keep that return value, mutate it, or replace it entirely with a different value.

Something to the effect of this:.

MethodInfo myMethod;
var plug = Nasal.InstanceMethod<MyReturn>(myMethod)
    .ReturnWrapped((object instance, MyReturn originalReturn, MyParam p1, MyParam p2) =>
    {
        // Do stuff, and either return `originalReturn` or some other value

        return ...
    });

[Keboo]

I dug into this a bit more (documenting here so I don't forget).

The reason this is not possible with the current API is because when using a plug, it only registers a prefix (BaseMethodHandler.Patch). To be able to what is described above would mean one of two things:

1. Implicitly set the CallOriginal flag so that the original plugged method still gets called. We would the need to invoke the user provided delegate and replace the return value with the user provided value.
2. What is being described is the Postfix behavior. We could provide an API for getting access to this. This is probably the better solution.