resolves #1127 add key documentation links to README

- also adds them to asciidoctor.org home page
Merge pull request #1153 from mojavelinux/issue-1127

resolves #1127 add key documentation links to README
remove some dead code
resolves #1068 make line comment before callout number configurable

- make line comment before callout number configurable
- add -- as built-in line comment to support Haskell, SQL, Ada, etc.
- add more tests for stripping line comment preceding callout number