resolves #718 use + and ++ as constrained and unconstrained passthrough

- change + to be constrained passthough (no monospaced formatting)
- change ++ to be unconstrained passthrough (no monospaced formatting)
  * same as $$
- change ` to be constrained monospace (no passthrough behavior)
- change `` to be unconstrained monospace (no passthrough behavior)
- remove inline attributes from inline constrained passthrough (i.e., +)
- introduce compat-mode document attribute to reenable legacy behavior
- add compat_mode property to document to set compatibility mode
- update compat file for compatibility with AsciiDoc Python
- update tests for both compatibility modes