Retain unreferenced option declarations in spec

[felixSchl]

The specification needs a change in type in order to carry along not only information about options and their arrangement among one another but also options that have been declared but were not referenced.
This will enable #72.

[felixSchl]

This will be irrelevant once the work outlined in #42 is done, as the definition of a "spec" changed dramatically and we can simply serialize "whenever" (i.e. pre or post any transformations, given that the arg has a fromForeign and toForeign instance.)