Merge pull request #258 from mojavelinux/explicit-subs

resolves #220 support explicit subs on blocks
resolves #187 complete masquerade functionality for blocks & paragraphs

- rework block parsing
- move block creation to a separate method
- add method to assign or generate block caption
- add set_attr method to block
- use sets where appropriate to improve speed
- cleanup two-line section matching
- optimize reader operations
- add flags to control compliance w/ AsciiDoc
- tests for masquerading, captions and other compliance