syntax highlighting broken with C++ and missing semicolon

[vvavrychuk]

OK:

After removing semicolon on line 4:

[ggrossetie]

Related to #686. If you think you can improve the language grammar, feel free to give it a try: master/syntaxes/Asciidoctor.json.

I believe that the root cause is that the syntax in a code block should be parsed as "plain text" but since we are using a set of regular expressions (i.e., context-free grammar), the content is still interpreted.