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/flexbox_helper.hpp

@@ -0,0 +1,45 @@
+#ifndef FTXUI_DOM_FLEXBOX_HELPER_HPP
+#define FTXUI_DOM_FLEXBOX_HELPER_HPP
+
+#include <vector>
+#include "ftxui/dom/flexbox_config.hpp"
+
+namespace ftxui {
+namespace flexbox_helper {
+
+struct Block {
+  // Input:
+  int min_size_x = 0;
+  int min_size_y = 0;
+  int flex_grow_x = 0;
+  int flex_grow_y = 0;
+  int flex_shrink_x = 0;
+  int flex_shrink_y = 0;
+
+  // Output:
+  int line;
+  int line_position;
+  int x = 0;
+  int y = 0;
+  int dim_x = 0;
+  int dim_y = 0;
+  bool overflow = false;
+};
+
+struct Global {
+  std::vector<Block> blocks;
+  FlexboxConfig config;
+  int size_x;
+  int size_y;
+};
+
+void Compute(Global& global);
+
+}  // namespace flexbox_helper
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_DOM_FLEXBOX_HELPER_HPP*/
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.