Mouse support. Fix & verify Webassembly support.

There was some undefined behavior to be fixed in the terminal input
parser.

The behavior of flush seems to have change. The fix was to invert '\0'
and std::flush.

include/ftxui/component/input.hpp

@@ -27,6 +27,11 @@ class Input : public Component {
   // Component implementation.
   Element Render() override;
   bool OnEvent(Event) override;
+
+ private:
+  bool OnMouseEvent(Event);
+  Box input_box_;
+  Box cursor_box_;
 };
 
 }  // namespace ftxui