Document how to provide memberwise initializers for Parsable* types

[natecook1000]

As pointed out in #120, there isn't any documentation describing how to manually create instances of ParsableArguments or ParsableCommand types, and the error message when using one created from calling the default init (e.g. let command = MyCommand()) indicates that the only way to create one is through parsing.

Instead, the Manual Parsing and Testing guide should cover adding a memberwise init as a way to create testable instances of the type, and the error should be updated to indicate that possibility.