resolves #717 disable single quotes as formatting marks for emphasized text

- disable single quotes as emphasis formatting marks by default
- introduce compat-mode document attribute to reenable legacy behavior
- add compat_mode property to document to set compatibility mode
- don't replace trailing single quote with right single smart quote
- introduce `' as the formatting mark to make a right single quote (rsquo)
- unescape escaped single quote explicitly
- update compat file for AsciiDoc Python