Only create one instance of sensor_module for ADS1x15

[shbatm]

Instead of requiring a separate sensor_module config section for each pin on an ADS1x15, this requires you to set the pins to use in a single module, and assigning the specific sensor pins in the sensor_inputs section--allowing the re-use of the Sensor class instance and preventing the issues seen in #254.

The ads1x15 sensor class has been updated to create an analog channel for each pin assigned in the config, up to all 4 available channels.

New config example:

sensor_modules:
  - name: ads_0
    module: ads1x15
    type: ADS1115
    pins:
      - 0
      - 1

sensor_inputs:
  - name: voltage_channel_0
    module: ads_0
    type: voltage
    pin: 0
    interval: 2

  - name: voltage_channel_1
    module: ads_0
    type: voltage
    pin: 1
    interval: 2

Fixes #254.

[jcsouthworth]

Thank you for posting this request - would fix the issue I am having. Hoping this gets merged somewhat soon!