Adding extensible, type-specific serialization for code blocks.

My goal was to allow special handling of specific code blocks.  I think this is a pretty succinct solution.  I toyed with the idea of using the ServiceLoader to automatically load any VerbatimSerializers registered on the classpath, but decided to go the more manual route.  Let me know if you want me to add some more automated extension loading -- or if you have any other questions or concerns about it.