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.

include/ftxui/screen/box.hpp

@@ -11,6 +11,8 @@ struct Box {
 
   static Box Intersection(Box a, Box b);
   bool Contain(int x, int y);
+  bool operator==(const Box& other) const;
+  bool operator!=(const Box& other) const;
 };
 
 }  // namespace ftxui