Request: MQ135 Air Quality Sensor (PPM) support

**Describe the problem you have/What new integration you would like**

I would like to request sensor integration for the MQ135 air quality gas sensor. This sensor provides 4 pins: 3.3v, GND, DOUT, AOUT.

**Please describe your use case for this integration and alternatives you've tried:**

I have tried using custom sensor integrations, however I am still learning C++ and Arduino coding. I have found multiple libraries online (in .ino form and .h/.cpp form), and have tried multiple ways to impliment it into ESPHome, but I have been unsuccessful. Each library appears to use its own formula for calibration. The most accurate, from what I have heard, exists here: https://github.com/GeorgK/MQ135 however, additional libraries are:
https://platformio.org/lib/show/2537/TroykaMQ (comments in russian) and https://os.mbed.com/users/ngomez/code/MQ135/

**Additional context**

This is a gas/VOC sensor and should provide a reading output in PPM (particles per million). It appears the AOUT is used to provide the data to the NodeMCU/ESP8266.