fix for passthrough, quote & anchor subs; register reference method

- multi-line passthrough, quote escaping, quoted xrefs and anchors:

- change the passthrough macro regexp to support multiple lines
- update unconstrained quoted text regexps to support escaping
- handle quoted anchors and xrefs
- tests for escaping all forms of quoted text

- add register method for ids and make references extensible:

- make references a hash of key=>collection pairs
- store id references in the :ids key of references
- catalog images and links
- remove deprecated anchor from abstract block
- use quoted strings for interpolation as it has better performance
- add to_s methods to make it easier to inspect the document structure

- add tests for blocks adjacent to paragraphs