🏗 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/ina3221/ina3221.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/sensor/sensor.h"
+#include "esphome/components/i2c/i2c.h"
+
+namespace esphome {
+namespace ina3221 {
+
+class INA3221Component : public PollingComponent, public i2c::I2CDevice {
+ public:
+  INA3221Component(uint32_t update_interval) : PollingComponent(update_interval) {}
+  void setup() override;
+  void dump_config() override;
+  void update() override;
+  float get_setup_priority() const override;
+
+  void set_bus_voltage_sensor(int channel, sensor::Sensor *obj) { this->channels_[channel].bus_voltage_sensor_ = obj; }
+  void set_shunt_voltage_sensor(int channel, sensor::Sensor *obj) {
+    this->channels_[channel].shunt_voltage_sensor_ = obj;
+  }
+  void set_current_sensor(int channel, sensor::Sensor *obj) { this->channels_[channel].current_sensor_ = obj; }
+  void set_power_sensor(int channel, sensor::Sensor *obj) { this->channels_[channel].power_sensor_ = obj; }
+  void set_shunt_resistance(int channel, float resistance_ohm);
+
+ protected:
+  struct INA3221Channel {
+    float shunt_resistance_{0.1f};
+    sensor::Sensor *bus_voltage_sensor_{nullptr};
+    sensor::Sensor *shunt_voltage_sensor_{nullptr};
+    sensor::Sensor *current_sensor_{nullptr};
+    sensor::Sensor *power_sensor_{nullptr};
+
+    bool exists();
+    bool should_measure_shunt_voltage();
+    bool should_measure_bus_voltage();
+  } channels_[3];
+};
+
+}  // namespace ina3221
+}  // namespace esphome