Add support for Xiaomi Mi Scale V1 and V2

https://github.com/esphome/feature-requests/issues/209

@tsunglung have all the rights and effort for this.

esphome/components/xiaomi_miscale/sensor.py

@@ -0,0 +1,36 @@
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.components import sensor, esp32_ble_tracker
+from esphome.const import CONF_BATTERY_LEVEL, CONF_MAC_ADDRESS, CONF_TEMPERATURE, \
+    UNIT_CELSIUS, ICON_THERMOMETER, UNIT_PERCENT, ICON_WATER_PERCENT, ICON_BATTERY, CONF_ID, \
+    CONF_MOISTURE, CONF_ILLUMINANCE, ICON_BRIGHTNESS_5, UNIT_LUX, CONF_CONDUCTIVITY, \
+    UNIT_MICROSIEMENS_PER_CENTIMETER, ICON_FLOWER
+
+DEPENDENCIES = ['esp32_ble_tracker']
+AUTO_LOAD = ['xiaomi_ble']
+UNIT_GRAMS = 'kg'
+CONF_WEIGHT = 'weight'
+ICON_SCALE = 'mdi:scale'
+
+xiaomi_miscale_ns = cg.esphome_ns.namespace('xiaomi_miscale')
+XiaomiMiscale = xiaomi_miscale_ns.class_('XiaomiMiscale', esp32_ble_tracker.ESPBTDeviceListener,
+                                         cg.Component)
+
+CONFIG_SCHEMA = cv.Schema({
+    cv.GenerateID(): cv.declare_id(XiaomiMiscale),
+    cv.Required(CONF_MAC_ADDRESS): cv.mac_address,
+    cv.Optional(CONF_WEIGHT): sensor.sensor_schema(UNIT_GRAMS, ICON_SCALE, 1),
+}).extend(esp32_ble_tracker.ESP_BLE_DEVICE_SCHEMA).extend(cv.COMPONENT_SCHEMA)
+
+
+def to_code(config):
+    var = cg.new_Pvariable(config[CONF_ID])
+    yield cg.register_component(var, config)
+    yield esp32_ble_tracker.register_ble_device(var, config)
+
+    cg.add(var.set_address(config[CONF_MAC_ADDRESS].as_hex))
+
+
+    if CONF_WEIGHT in config:
+        sens = yield sensor.new_sensor(config[CONF_WEIGHT])
+        cg.add(var.set_weight(sens))