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

examples/dom/style_underlined.cpp

@@ -9,14 +9,12 @@
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
-  // clang-format off
-  auto document =
-    hbox(
-      text(L"This text is "),
-      text(L"underlined") | underlined,
-      text(L". Do you like it?")
-    );
-  // clang-format on
+  auto document =  //
+      hbox({
+          text(L"This text is "),
+          text(L"underlined") | underlined,
+          text(L". Do you like it?"),
+      });
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
   Render(screen, document.get());