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/size.cpp

@@ -4,8 +4,8 @@
 #include <string>                   // for string, to_string
 #include <utility>                  // for move
 
-#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;