Add VEML7700 Light Level Sensor

[mschlenstedt]

Add VEML7700 Light sensor, based on library from Adafruit. Config example:

sensor_modules:
- chip_addr: 0x10
  module: veml7700
  name: veml7700_1
  gain: 0.125
  integration_time: 25
sensor_inputs:
- digits: 4
  interval: 30
  module: veml7700_1
  name: illuminance_veml7700
  type: lux_corrected
  retain: true

The module also implements the correction formula mentioned in the datasheet https://www.vishay.com/docs/84323/designingveml7700.pdf for lux values > 1000. This is not supported by the original adafruit library yet. Use type "lux_corrected" for the correted value.