🏗 Merge C++ into python codebase (#504)

## Description:

Move esphome-core codebase into esphome (and a bunch of other refactors). See https://github.com/esphome/feature-requests/issues/97

Yes this is a shit ton of work and no there's no way to automate it :( But it will be worth it 👍

Progress:
- Core support (file copy etc): 80%
- Base Abstractions (light, switch): ~50%
- Integrations: ~10%
- Working? Yes, (but only with ported components).

Other refactors:
- Moves all codegen related stuff into a single class: `esphome.codegen` (imported as `cg`)
- Rework coroutine syntax
- Move from `component/platform.py` to `domain/component.py` structure as with HA
- Move all defaults out of C++ and into config validation.
- Remove `make_...` helpers from Application class. Reason: Merge conflicts with every single new integration.
- Pointer Variables are stored globally instead of locally in setup(). Reason: stack size limit.

Future work:
- Rework const.py - Move all `CONF_...` into a conf class (usage `conf.UPDATE_INTERVAL` vs `CONF_UPDATE_INTERVAL`). Reason: Less convoluted import block
- Enable loading from `custom_components` folder.

**Related issue (if applicable):** https://github.com/esphome/feature-requests/issues/97

**Pull request in [esphome-docs](https://github.com/esphome/esphome-docs) with documentation (if applicable):** esphome/esphome-docs#<esphome-docs PR number goes here>

## Checklist:
  - [ ] The code change is tested and works locally.
  - [ ] Tests have been added to verify that the new code works (under `tests/` folder).

If user exposed functionality or configuration variables are added/changed:
  - [ ] Documentation added/updated in [esphomedocs](https://github.com/OttoWinter/esphomedocs).

esphome/components/xiaomi_mijia/xiaomi_mijia.h

@@ -0,0 +1,51 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/sensor/sensor.h"
+#include "esphome/components/esp32_ble_tracker/esp32_ble_tracker.h"
+#include "esphome/components/xiaomi_ble/xiaomi_ble.h"
+
+#ifdef ARDUINO_ARCH_ESP32
+
+namespace esphome {
+namespace xiaomi_mijia {
+
+class XiaomiMijia : public Component, public esp32_ble_tracker::ESPBTDeviceListener {
+ public:
+  XiaomiMijia(esp32_ble_tracker::ESP32BLETracker *parent, uint64_t address)
+      : ESPBTDeviceListener(parent), address_(address) {}
+
+  bool parse_device(const esp32_ble_tracker::ESPBTDevice &device) override {
+    if (device.address_uint64() != this->address_)
+      return false;
+
+    auto res = xiaomi_ble::parse_xiaomi(device);
+    if (!res.has_value())
+      return false;
+
+    if (res->temperature.has_value() && this->temperature_ != nullptr)
+      this->temperature_->publish_state(*res->temperature);
+    if (res->humidity.has_value() && this->humidity_ != nullptr)
+      this->humidity_->publish_state(*res->humidity);
+    if (res->battery_level.has_value() && this->battery_level_ != nullptr)
+      this->battery_level_->publish_state(*res->battery_level);
+    return true;
+  }
+
+  void dump_config() override;
+  float get_setup_priority() const override { return setup_priority::DATA; }
+  void set_temperature(sensor::Sensor *temperature) { temperature_ = temperature; }
+  void set_humidity(sensor::Sensor *humidity) { humidity_ = humidity; }
+  void set_battery_level(sensor::Sensor *battery_level) { battery_level_ = battery_level; }
+
+ protected:
+  uint64_t address_;
+  sensor::Sensor *temperature_{nullptr};
+  sensor::Sensor *humidity_{nullptr};
+  sensor::Sensor *battery_level_{nullptr};
+};
+
+}  // namespace xiaomi_mijia
+}  // namespace esphome
+
+#endif