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.

src/ftxui/dom/blink.cpp

@@ -23,7 +23,6 @@ class Blink : public NodeDecorator {
   }
 };
 
-
 /// @brief The text drawn alternates in between visible and hidden.
 /// @ingroup dom
 Element blink(Element child) {