Lots of changes to RDF(S) entailment.

* [OPTION_EXT,NEW] Added several predicates:
** default_otpion/[4,5] ensuring that an option is included in an options list.
** replace_option/5 replacing an option with a concrete value.
** update_option/[4,5] update an option with a predicate applied to the previous value.
* [PRINT_EXT,NEW] Added print_proof/[2,3].
* [RDF_AXIOM,NEW] Rewrote this entire module.
** Implemented all RDF(S) entailment rules.
** Implemented blank node to literal mapping.
** Added materialization with debug messages for length-1 derivations (should be length-n).
** Added parse trees representing proofs.
* [RDF_GRAPH,NEW] Added rdf_triple/4 allowing easy triple component to-from triple translations.
* [RDF_NAME,NEW] Added rdf_triple_name/2 wrapper around /4 variant.
* [TREES,NEW] Added print_tree/[2,3].