Add support for nxxm.

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

src/ftxui/dom/hflow.cpp

@@ -0,0 +1,59 @@
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+class HFlow : public Node {
+ public:
+  HFlow(Elements children) : Node(std::move(children)) {}
+  ~HFlow() {}
+
+  void ComputeRequirement() override {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    requirement_.flex.x = 1;
+    requirement_.flex.y = 1;
+    for(auto& child : children)
+      child->ComputeRequirement();
+  }
+
+  void SetBox(Box box) override {
+    Node::SetBox(box);
+
+    // The position of the first component.
+    int x = box.x_min;
+    int y = box.y_min;
+    int y_next = y; // The position of next row of elements.
+
+    for (auto& child : children) {
+      Requirement requirement = child->requirement();
+
+      // Does it fit the end of the row?
+      if (x + requirement.min.x > box.x_max) {
+        // No? Use the next row.
+        x = box.x_min;
+        y = y_next;
+      }
+
+      // Does the current row big enough to contain the element?
+      if (y + requirement.min.y > box.y_max + 1)
+        break; // No? Ignore the element.
+
+      Box children_box;
+      children_box.x_min = x;
+      children_box.x_max = x + requirement.min.x - 1;
+      children_box.y_min = y;
+      children_box.y_max = y + requirement.min.y - 1;
+      child->SetBox(children_box);
+
+      x = x + requirement.min.x;
+      y_next = std::max(y_next, y + requirement.min.y);
+    }
+  }
+};
+
+std::unique_ptr<Node> hflow(Elements children) {
+  return std::make_unique<HFlow>(std::move(children));
+}
+
+};  // namespace ftxui