Use shared_ptr instead of unique_ptr for elements.

This allow users to pass it into initializer list. Then clang-format
will produce 'acceptable' indentations.

This fixes:
https://github.com/ArthurSonzogni/FTXUI/issues/18

src/ftxui/dom/node.cpp

@@ -9,8 +9,7 @@ namespace ftxui {
 using ftxui::Screen;
 
 Node::Node() {}
-Node::Node(std::vector<std::unique_ptr<Node>> children)
-    : children(std::move(children)) {}
+Node::Node(Elements children) : children(std::move(children)) {}
 Node::~Node() {}
 
 void Node::ComputeRequirement() {