More consilidating the recent DCG-based code cleanup.

* [HTML_PL_TERM,FIX] Atoms would be written with extra quotes.
* [RDF_HTML_TABLE,NEW] Added rdf_html_table//3 for non-standard header rows. rdf_html_table//2 uses this now as well.
* [RDF_PARSE,FIX] Literals were not parsed because they were not included in the main parse clauses.
* [URI_EXT,FIX] Stronger typecheck for is_image_url/1.