Add support for nxxm.

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

src/ftxui/dom/util.cpp

@@ -0,0 +1,33 @@
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+Element nothing(Element element) {
+  return element;
+}
+
+Decorator compose(Decorator a, Decorator b) {
+  return [
+    a = std::move(a),
+    b = std::move(b)
+  ](Element element) {
+    return b(a(std::move(element)));
+  };
+}
+
+Decorator operator|(Decorator a, Decorator b) {
+  return compose(a, b);
+}
+
+Elements operator|(Elements es, Decorator d) {
+  Elements output;
+  for (auto& it : es)
+    output.push_back(std::move(it) | d);
+  return output;
+}
+
+Element operator|(Element e, Decorator d) {
+  return d(std::move(e));
+}
+
+}  // namespace ftxui