osyncstream support

[nikolay-govorov]

In C++ 20 add syncstream - wrappers that allows to synchronize output into a single stream. Due to the fact that it wraps the stream, rbuf does not return a real pointer and rang believes that the stream does not support color at all.

For example:

#include <iostream>
#include <syncstream>

#include "rang.hpp"

int main() {
  using namespace std;
  using namespace rang;

  std::osyncstream sout(std::cout);

  cout << fg::red << "Rang styled text!!" << style::reset << endl; // colofull
  sout << fg::red << "Rang styled text!!" << style::reset << endl; // plain text

  return 0;
}

It would be cool to specify rand to work with it. The instance of the class (msg in the example) has the get_wrapped method, which return the correct buffer, which can be checked correctly.

What do you think about it? I'm not a very experienced C++ programmer, but I could try to send a PR if you think it's a good idea.