Feature Request: Emscripten support

[vittorioromeo]

Would be nice to get even (partial) support for Emscripten -- I currently only need cpptrace::generate_trace().print() in my project, and I have to #ifdef for Emscripten like this:

#ifdef SFML_SYSTEM_EMSCRIPTEN
    char callstack[4096];
    emscripten_get_callstack(EM_LOG_C_STACK, callstack, sizeof(callstack));
    std::puts(callstack);
#else
    cpptrace::generate_trace().print();
#endif

Would be nice if cpptrace handled this automatically :)