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/separator.cpp

@@ -2,8 +2,8 @@
 #include <ftxui/screen/screen.hpp>  // for Full, Screen
 #include <memory>                   // for allocator
 
-#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;