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

- change + to be constrained passthough (no monospaced formatting)
  * recognize role "x-" to reenable legacy behavior
- change ++ to be unconstrained passthrough (no monospaced formatting, same as $$)
  * recognize role "x-" to reenable legacy behavior
- add role to unconstrained passthroughs (except for pass:[] macro)
- change ` to be constrained monospace (no passthrough behavior)
- change `` to be unconstrained monospace (no passthrough behavior)
- use compat-mode document attribute to reenable legacy behavior
- update compat file for compatibility with AsciiDoc Python
- update tests for both compatibility modes
- fix marked text in compat file
resolves #1034 properly restore nested passthroughs