Fix additional bugs uncovered by rendering user guide

- Fix the source of an infinite loop when processing a paragraph w/ no content
- Make sure indented nested labeled lists are found
- Fix list items that were overrunning following content (too greedy)
- Fix list item from eating title on following detached block
- Prevent empty dd element from appearing
- Support brackets before link macro and round brackets around it
- Set asciidoctor attribute for use with ifdef

With the exception of tables, we can now render the AsciiDoc User Guide without breaking the structure.

This pull request is based on the previous one. I can rebase as necessary.