Add demo integration (#2085)

esphome/components/demo/demo_binary_sensor.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/binary_sensor/binary_sensor.h"
+
+namespace esphome {
+namespace demo {
+
+class DemoBinarySensor : public binary_sensor::BinarySensor, public PollingComponent {
+ public:
+  void setup() override { this->publish_initial_state(false); }
+  void update() override {
+    bool new_state = last_state_ = !last_state_;
+    this->publish_state(new_state);
+  }
+
+ protected:
+  bool last_state_ = false;
+};
+
+}  // namespace demo
+}  // namespace esphome