Tweak whitespace in a lot of areas for readability and make style tweaks to variable and key names where possible (i.e., not dictated by Asciidoc's spec).
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 'origin/jm_whitespace_and_style_tweaks' into whitespace

Conflicts:
	lib/asciidoctor/abstract_node.rb
Merge remote-tracking branch 'mojavelinux/list-reader-rework' into mojave