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_config.cpp

@@ -0,0 +1,40 @@
+#include "ftxui/dom/flexbox_config.hpp"
+
+namespace ftxui {
+
+FlexboxConfig& FlexboxConfig::Set(FlexboxConfig::Direction d) {
+  this->direction = d;
+  return *this;
+}
+
+FlexboxConfig& FlexboxConfig::Set(FlexboxConfig::Wrap w) {
+  this->wrap = w;
+  return *this;
+}
+
+FlexboxConfig& FlexboxConfig::Set(FlexboxConfig::JustifyContent j) {
+  this->justify_content = j;
+  return *this;
+}
+
+FlexboxConfig& FlexboxConfig::Set(FlexboxConfig::AlignItems a) {
+  this->align_items = a;
+  return *this;
+}
+
+FlexboxConfig& FlexboxConfig::Set(FlexboxConfig::AlignContent a) {
+  this->align_content = a;
+  return *this;
+}
+
+FlexboxConfig& FlexboxConfig::SetGap(int x, int y) {
+  this->gap_x = x;
+  this->gap_y = y;
+  return *this;
+}
+
+}  // namespace ftxui
+
+// 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.