Feature Description: Dynmap should use environment variables as a way to override the values configured in configuration.txt

• Additional context: I like to keep my configuration values checked into a git repo but I am currently having to exclude dynmap's configuration.txt since it contains the credentials to my SQL server. Most other mods that contain secrets provide a way to set them via environment variables (or a .env file) to allow for the secret to not be hard coded in the configuration file. This also makes it easy to configure these values in docker, which is how I run my MC server.