Add environment variables to config.json

[MichaelAceAnderson]

Describe the solution you'd like
I would like to be able to use environment variables interpolated in the config.json file.
Example:

{
  "rcon": {
    "password": "${RCON_PASSWORD}"
  }
}

This could make the config.json more reusable, but also make it much simpler to Dockerize a open.mp server.
In addition, I would rather not store my passwords on my repositories.