Add support for nxxm.

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

src/ftxui/dom/gauge.cpp

@@ -0,0 +1,39 @@
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+using namespace ftxui;
+
+static wchar_t charset[] = L"  ▏▎▍▌▋▊▉█";
+
+class Gauge : public Node {
+ public:
+  Gauge(float progress) : progress_(progress) {}
+  ~Gauge() override {}
+
+  void ComputeRequirement() override {
+    requirement_.flex.x = 1;
+    requirement_.min.y = 1;
+  }
+
+  void Render(Screen& screen) override {
+    float y = box_.y_min;
+    float limit = box_.x_min + progress_ * (box_.x_max - box_.x_min + 1);
+    int limit_int = limit;
+    int x = box_.x_min;
+    while (x < limit_int)
+      screen.at(x++, y) = charset[9];
+    screen.at(x++, y) = charset[int(9*(limit-limit_int))];
+    while (x <= box_.x_max)
+      screen.at(x++, y) = charset[0];
+  }
+ private:
+  float progress_;
+};
+
+std::unique_ptr<Node> gauge(float progress) {
+  return std::make_unique<Gauge>(progress);
+}
+
+};  // namespace ftxui