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/html_like.cpp

@@ -44,7 +44,7 @@ int main(int argc, const char* argv[]) {
             paragraph(" A spinner "), spinner(6, i / 10)) |
         border;
 
-    auto screen = Screen::Create(Dimension::Full());
+    auto screen = Screen::Create(Dimension::Fit(document));
     Render(screen, document);
     std::cout << reset_position;
     screen.Print();