Add colored border. (#595)

This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/564

examples/dom/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(DIRECTORY_LIB dom)
 
 example(border)
+example(border_colored)
 example(border_style)
 example(color_gallery)
 example(color_info_palette256)

examples/dom/border_colored.cpp

@@ -0,0 +1,40 @@
+#include <ftxui/dom/elements.hpp>  // for operator|, text, Element, Fit, borderDouble, borderHeavy, borderLight, borderRounded, vbox
+#include <ftxui/screen/screen.hpp>  // for Screen
+#include <iostream>                 // for endl, cout, ostream
+#include <memory>                   // for allocator
+
+#include "ftxui/dom/node.hpp"      // for Render
+#include "ftxui/screen/color.hpp"  // for ftxui
+
+int main(int argc, const char* argv[]) {
+  using namespace ftxui;
+
+  auto make_boxed = [] {
+    return vbox({
+        text("borderLight") | borderStyled(LIGHT, Color::Red),
+        text("borderDashed") | borderStyled(DASHED, Color::Green),
+        text("borderHeavy") | borderStyled(HEAVY, Color::Blue),
+        text("borderDouble") | borderStyled(DOUBLE, Color::Yellow),
+        text("borderRounded") | borderStyled(ROUNDED, Color::Cyan),
+    });
+  };
+
+  auto document = hbox({
+                      make_boxed(),
+                      separator() | color(Color::Red),
+                      make_boxed(),
+                      separator() | color(Color::Red),
+                      make_boxed(),
+                  }) |
+                  borderStyled(ROUNDED, Color::Red);
+
+  auto screen =
+      Screen::Create(Dimension::Fit(document), Dimension::Fit(document));
+  Render(screen, document);
+  screen.Print();
+  std::cout << std::endl;
+}
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.