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

include/ftxui/dom/elements.hpp

@@ -109,6 +109,8 @@ Element bgcolor(const LinearGradient&, Element);
 Decorator focusPosition(int x, int y);
 Decorator focusPositionRelative(float x, float y);
 Element automerge(Element child);
+Decorator hyperlink(std::string link);
+Element hyperlink(std::string link, Element child);
 
 // --- Layout is
 // Horizontal, Vertical or stacked set of elements.

include/ftxui/screen/screen.hpp

@@ -1,8 +1,9 @@
 #ifndef FTXUI_SCREEN_SCREEN_HPP
 #define FTXUI_SCREEN_SCREEN_HPP
 
+#include <cstdint>  // for uint8_t
 #include <memory>
-#include <string>  // for string, allocator, basic_string
+#include <string>  // for string, basic_string, allocator
 #include <vector>  // for vector
 
 #include "ftxui/screen/box.hpp"       // for Box
@@ -20,6 +21,10 @@ struct Pixel {
   // like: a⃦, this can potentially contains multiple codepoitns.
   std::string character = " ";
 
+  // The hyperlink associated with the pixel.
+  // 0 is the default value, meaning no hyperlink.
+  uint8_t hyperlink = 0;
+
   // Colors:
   Color background_color = Color::Default;
   Color foreground_color = Color::Default;
@@ -99,6 +104,11 @@ class Screen {
   Cursor cursor() const { return cursor_; }
   void SetCursor(Cursor cursor) { cursor_ = cursor; }
 
+  // Store an hyperlink in the screen. Return the id of the hyperlink. The id is
+  // used to identify the hyperlink when the user click on it.
+  uint8_t RegisterHyperlink(std::string link);
+  const std::string& Hyperlink(uint8_t id) const;
+
   Box stencil;
 
  protected:
@@ -106,6 +116,7 @@ class Screen {
   int dimy_;
   std::vector<std::vector<Pixel>> pixels_;
   Cursor cursor_;
+  std::vector<std::string> hyperlinks_ = {""};
 };
 
 }  // namespace ftxui