Add support for nxxm.

[nxxm](https://nxxm.github.io)

src/ftxui/screen/terminal.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <sys/ioctl.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "ftxui/screen/terminal.hpp"
+
+namespace ftxui {
+
+Terminal::Dimensions Terminal::Size() {
+#ifdef __EMSCRIPTEN__
+  return Dimensions{80,43};
+#else
+  winsize w;
+  ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);
+  std::cerr << w.ws_col << "," << w.ws_row << std::endl;
+  return Dimensions{w.ws_col, w.ws_row};
+#endif
+}
+
+} // namespace ftxui