syntax highlighter of source code block fails when using options

[prudhomm]

consider this code

[source,cpp]
----
#include <iostream> // <1>

int main() { // <2>
    std::cout << "Hello, world" << std::endl; // <3>
    return 0; // <4>
}
----

this is highlighted properly as follows in vscode:

However if I do something like that, the highlighter does not recognize that we are in c++ anymore

[ggrossetie]

As mentioned in #686, we currently rely on a TextMate grammar but it cannot properly parse AsciiDoc.