Add support for password for input element. (#158)

This fixes:
https://github.com/ArthurSonzogni/FTXUI/issues/139

CC:@Creapermann

include/ftxui/component/component_options.hpp

@@ -50,6 +50,9 @@ struct InputOption {
   /// Called when the user presses enter.
   std::function<void()> on_enter = [] {};
 
+  /// Obscure the input content using '*'.
+  bool password = false;
+
   Ref<int> cursor_position = 0;
 };