Add environment variables to config.json

[MichaelAceAnderson]

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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.