Feature: hyperlink support. (#665)

See the [OSC 8 page](https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda).
FTXUI support proposed by @aaleino in [#662](https://github.com/ArthurSonzogni/FTXUI/issues/662).

API:
```cpp
auto link = text("Click here") | hyperlink("https://github.com/FTXUI")
```

Fixed:https://github.com/ArthurSonzogni/FTXUI/issues/662

examples/dom/CMakeLists.txt

@@ -27,6 +27,7 @@ example(style_bold)
 example(style_color)
 example(style_dim)
 example(style_gallery)
+example(style_hyperlink)
 example(style_inverted)
 example(style_strikethrough)
 example(style_underlined)

examples/dom/style_gallery.cpp

@@ -19,7 +19,8 @@ int main() {
       text("blink")              | blink                , text(" ") ,
       text("strikethrough")      | strikethrough        , text(" ") ,
       text("color")              | color(Color::Blue)   , text(" ") ,
-      text("bgcolor")            | bgcolor(Color::Blue) ,
+      text("bgcolor")            | bgcolor(Color::Blue) , text(" ") ,
+      text("hyperlink")          | hyperlink("https://github.com/ArthurSonzogni/FTXUI"),
     });
   // clang-format on
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));

examples/dom/style_hyperlink.cpp

@@ -0,0 +1,25 @@
+#include <ftxui/dom/elements.hpp>  // for text, operator|, bold, Fit, hbox, Element
+#include <ftxui/screen/screen.hpp>  // for Full, Screen
+#include <memory>                   // for allocator
+
+#include "ftxui/dom/node.hpp"      // for Render
+#include "ftxui/screen/color.hpp"  // for ftxui
+
+int main() {
+  using namespace ftxui;
+  auto document =  //
+      hbox({
+          text("This text is an "),
+          text("hyperlink") | hyperlink("https://www.google.com"),
+          text(". Do you like it?"),
+      });
+  auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
+  Render(screen, document);
+  screen.Print();
+
+  return 0;
+}
+
+// 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.