Implement flexbox (#277)

This implement the flexbox elements, following the HTML one.

Built from them, there is also the following elements:
- `paragraph`
- `paragraphAlignLeft`
- `paragraphAlignRight`
- `paragraphAlignCenter`
- `paragraphAlignJustify`

This is a breaking change.

examples/dom/hflow.cpp

@@ -1,11 +1,12 @@
 #include <stddef.h>                // for size_t
+#include <stdio.h>                 // for getchar
 #include <ftxui/dom/elements.hpp>  // for operator|, size, Element, text, hcenter, Decorator, Fit, WIDTH, hflow, window, EQUAL, GREATER_THAN, HEIGHT, bold, border, dim, LESS_THAN
 #include <ftxui/screen/screen.hpp>  // for Full, Screen
 #include <memory>                   // for allocator, shared_ptr
 #include <string>  // for operator+, to_string, char_traits, string
 
-#include "ftxui/dom/node.hpp"    // for Render
-#include "ftxui/screen/box.hpp"  // for ftxui
+#include "ftxui/dom/flexbox_config.hpp"  // for ftxui
+#include "ftxui/dom/node.hpp"            // for Render
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
@@ -44,6 +45,7 @@ int main(int argc, const char* argv[]) {
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
   Render(screen, document);
   screen.Print();
+  getchar();
 
   return 0;
 }