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/dbox.cpp

@@ -9,19 +9,16 @@
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
-  // clang-format off
-  auto document =
-    dbox(
-      vbox(
-        text(L"line_1"),
-        text(L"line_2"),
-        text(L"line_3"),
-        text(L"line_4"),
-        text(L"line_5")
-      ) | border,
-      text(L"overlay") | border | center
-    );
-  // clang-format on
+  auto document = dbox({
+      vbox({
+          text(L"line_1"),
+          text(L"line_2"),
+          text(L"line_3"),
+          text(L"line_4"),
+          text(L"line_5"),
+      }) | border,
+      text(L"overlay") | border | center,
+  });
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
   Render(screen, document.get());