Support for .yaml config files / --config flag

[keebrev]

First of all, I like the fact this is dockerized! Although, I am missing one key-feature to make this work for our dev-environment. We are using multiple tunnels simultaneously and have this configured in a ngrok.yaml file.

I'd love support for the --config flag so I'd be able to use it like this (or something similar):

# SNIPPET OF docker-compose.yaml file
  ngrok:
    image: shkoliar/ngrok:latest
    ports:
      - 4551:4551
    volumes:
      - ./backend/example_task_handler/ngrok.yaml:/ngrok.yaml
    links:
      - task_handler_one
      - task_handler_two
    environment:
      - PARAMS=start --config=ngrok.yaml --all