rework list reading to be more compliant & compact

- rework lists, specifically dlists to follow proper scanning rules
- merge logic for handling labeled and outline lists
- more robust list continuations
- fix bug in folding first block of list item
- add :olist to LIST_CONTEXTS
- create constant NESTABLE_LIST_CONTEXTS to exclude :olists
- fix missing pipe in "any block" regexp
- don't capture endlines in :dlist regexp
- add a "text only" scan mode to next_block
- add method to reader to consume line comments
- many more tests to ensure compliant list behavior
Merge remote-tracking branch 'mojavelinux/list-reader-rework' into mojave