Allow nullable value types to be built as null

[Doug-Murphy]

This idea came from #86. It would be helpful to be able to allow nullable value types to be generated as null instead of falling back to their non-nullable equivalents.

The default behavior should remain as-is. That is to say that a nullable value type will be generated with its non-null equivalent's default value. Ex: int? foo will be generated as 1.

There should be two levels of this supported:

1. Specific nullable types.
2. All nullable types.

I have already begun work on this issue.

[PureKrome]

Now fixed via #107